Is track run working with a track different from 400m ?

400 only unless there are undocumented changes.

https://support.garmin.com/en-US/?faq=eH8V1VwjN338uo8C5PgDJ7

Yes it's what I read also but I read somewhere that the watch was detecting the track length after 3 laps so I don't understand if it's relying on 400m length or if it is evaluating the distance after some laps.

According to DC Rainmaker, Garmin claimed this:

Garmin says that it takes approximately two laps for them to work out the details of your track. They also say their algorithm can correctly detect any track as long as it has two parallel straightaways, and either a single or double-radius curve at two ends.

So where is the truth? Someone was lying. DC Rainmaker or Garmin.  Or it works only on FR745 which I don't believe that...

No one is "lying".  I've never seen any documentation that claims any track distance.  Garmin's support page on this feature is very clear that a 400m standard track is required. (https://support.garmin.com/en-US/?faq=eH8V1VwjN338uo8C5PgDJ7) as well as the owner's manuals for the watches that support the Track Run mode.

So if someone writes that it works on any track, it is not a lie?? If no one is lying, then it must work on any track :-).

https://www.dcrainmaker.com/2020/09/garmin-forerunner-745-depth-review.html

Garmin says...

All I'm saying is that the source documentation from Garmin states 400m tracks.  I've read the 745 review from DC and you can take it to mean any 400m track of a specific shape.  Certainly, his evaluation of the feature on the 745 and 55 were on a 400m oval track.

I know that manual says 400m only. I was talking about the review. But F7 has very good GPS, that I get absolutely accurate length even on 200m oval. F6Pro was a bad joke...

Garmin support said to me that only shape is important, distance will be the distance of the track if it’s anything besides 400m

https://postimg.cc/1fKk6qcK

There is no one in this forum, who makes Track Run work for other lengths than 400m. Even the run on another track doesn't create file in RunningTrack folder in watch, which is created on 400m track...

My experience with live Garmin support is same as if I call to mobile operator...

I have a track that is 380m and an other that isn’t 400, don’t remember how long though.

so there are people who runs other tracks

well i have not tried those with track activity unfortunately.

and yes, I’ve noticed that with the support, it’s sad.