Thanks Stephan for all your efforts!
Situation has changed - I shall do that IF it fails next time, not WHEN.
Why? Because being puzzled by the only change being done to antenna and making it actively worse after recrimping, I went for just-in-case firmware upgrade on main antenna and three small ones.
Not a single issue since then, lost three pings out of 8000 and did not care about it at all.
So far all this points to wireless AP issue, nothing to do with B&R. Still makes me wonder why could I ping the blasted things when they did fail, which is something I will definitely test by direct connection if this ever happens again.