Uber lab trials only in low tier maps?

Just wondering if I'm super unlucky or if this is how it's supposed to work. I was doing tier 1 to tier 5 maps for the longest time and was finding a trial once every blue moon. Once I moved to t7+, I never found a single trial, other than with the prophecy that forces a trial to appear in your map. Finally found my 6th trial at level 87 (after 17 found) running a random t2 for twinned boss challenge. Since level 83 (currently 89), running t7-t9, I have not found one in days. Dozens if not hundreds of maps later and still no trials.

Do they only spawn in low tier maps? How else are you supposed to get the keys if they don't spawn in "end game" maps? (I know t7-t9 is not end game but no better word for it).

Definitely not, got 3 in a row just today running red maps.

I got both zana and a trial in each of two consecutive plaza maps a few days ago. You're just having bad luck.

I think its random

I run T1-6 from ~lvl 70, currently lvl 81 and haven´t found single one. Currently I dont believe solo players like me who make 3-4 chars to mid 80s per season are expected to run Endgame lab