Assume it is just aerodynamics. PA46 filter is forward facing. Alternate is backward facing.
My air filter is forward facing.
The alternate air is taken from under the cowling and is the air that’s been warmed up by the passage through the cylinder fins.
I experienced air filter icing in my TB20 and actually saw the ice on it after landing.
My guess is that if the TB20 was FIKI approved, the POH’s Icing section would tell you to switch to alternate, too. Right now it doesn’t have such a section (or if it does, it just says “leave the area immediately”) which is why it’s not telling you anything about alternate air in icing conditions. Right?
There are FIKI approved TB20s with TKS deicing. I flew one many years ago and I don’t recall any particular instruction in the POH to switch to alternate air in icing conditions.
My guess is that if the TB20 was FIKI approved, the POH’s Icing section would tell you to switch to alternate, too. Right now it doesn’t have such a section (or if it does, it just says “leave the area immediately”) which is why it’s not telling you anything about alternate air in icing conditions. Right?
Most likely, yes.
So I wonder what the TKS AFMS says…
Switching to alternate air in any potential icing conditions is no big deal. But you don’t want to do it too much because the engine ingests so much dust and the oil analysis figures for the soft metals (e.g. copper) go way up.
I found my copy of the deiced TB20 POH. Indeed it says that engine alternate air must be on whenever the OAT (indicated) is below +4°C and there is visible moisture.
I wonder why I have never seen air intake icing, ever. I am using the same sort of air filter as anybody else.
However when in icing conditions I always have the prop TKS on max, and that will spray onto the air filter also.
So I would like to share my experience yesterday when I flew back from Hungary yesterday with my second (light) encounter with icing.
About 2/3 of track was over ground with IFR conditions on ground (mist with low visibility).
The first 30 minutes of my flight were in fact…quite interesting and a real learning curve….
I have no anti ice or de-ice equipment whatsoever on my plane so any (continued flying into icing conditions is a no-go). There was no icing forecast except in the North east of Hungary
For those who know Hungary in November you can get hi-pressure systems that lock-in for days with hi-humidity, fog and cold weather….
So yesterday was one of those days, I was monitoring weather for days to see what the evolution was going to be…
But at the end its only the day of flight, you will have to make the call. OAT was 3 degrees. O degrees where I took off about 4000 Ft (according gramet), however closer to Austria the 0 degree level rapidly dropped according the gramet. After Linz again to rise again above 8000 Feet
I decided to wait till midday (for things to heat up a bit) to start so I still would be able to land VFR at my destination in Belgium..
The METAR for Pecs showed a ceiling/overcast at 8000 feet… Before take off I was able to get a Pirep from some guys who came from Szeged with C172 and they told me at 5000Ft there was no ice…however with several layers of cloud.. (how they remained VFR I have no clue looking back on that.. …)
After my marginal VFR departure I picked up my IFR clearance with a level off at 8000 Ft (transition altitude is 9000 Ft in Hungary)….I was hoping to be on top at about 6000Ft..
I entered IMC then 2000, 3000ft got my IFR climb clearance, 4000ft, 5000ft…luckily still no ice just water droplets from fog, 6000 feet still full IMC, 7000 ft still IMC, OAT on my old Scot probe remained a bit below 0 degrees and now the water droplets..started to become less liquid with traces of ice on the wings….8000 feet still full IMC I started to get a bit worried. I had passed the lake Balaton now and I figured that I was not going to continue all the way along Vienna over the mountains in these conditions….so I asked for FL100..and see what would happen. If by that altitude I was not on top, I was going to turn back….(The day before the Austrian Met service mentioned that above 1700 Meters the mountains would be in the clear….While I requesting further climb to FL100 I noted some more brighter light behind my back where the sun was. That pushed my motivation..I was hand flying the aircraft to feel what the airframe was doing I sensed an increased vibration which to my understanding came from the prop as on the wings there was only light ripe visible…
Anyway the feeling when suddenly you see the sun on go on top is like entering paradise…;-). After leveling off I saw that sun started to burn away little ice slowly….The rest of the flight went rather uneventful…
My point is and there I support also Peter…What is known icing? Here there was no frontal system it was humidity, fog clouds etc..I think the most important thing I learned from flying yesterday always leave yourself an escape option and there is no replacement for displacement (engine power)..Now a hot prop and a Turbo engine would already help a lot…. 
cheers
It might be interesting to get a skew-t for the area – here
Select Europe and Skew-T
and then click on the desired station e.g. ETGB produces this for the latest data:
which is obviously a very low cloud layer, tops about 1500ft (it is very hard to interpolate the vertical axis that far down), and blue skies above
The worst ice I ever picked up by far was in smooth stratus, base 1500ft, tops 4000ft, over EGMD. 30mm in 5 minutes 
The aircraft was barely flyable on max power (TKS de-iced prop) at 1000ft.
