I work a lot in extended just intonation, and thus with huge octave divisions (I use 12000EDO with HEJI accidentals). With smaller divisions Dorico seems to handle trills just fine, but something breaks with larger divisions. In the “Interval” dialogue of the trill only a small minority of the possible accidentals are selectable, with the rest of them displaying +0/(number of steps). Sometimes, you can force Dorico to find the correct note by changing the key signature, copying another trill, or inserting a grace note, but in most cases there are just some notes which cannot be selected.
This bug has been in program for years by now (at least since 3.5), but I’ve been holding off posting about it since it’s such a niche issue (assuming it’s been a known issue). It would be nice if it eventually got patched 
