So, I recently started a fresh playthrough of this game after getting through all available content way back when Gawnfall was still the newest update, and I'm pretty sure I found an odd bug.
In Govern Yhilin III (Just after Zirantia trip, Airship Negotiation section), you're normally supposed to receive 1 additional Trin copy slot by default, plus one more at 50, 75 and 100 RP. Yet to my confusion I only received one slot (taking me from 7 to 8), despite Trin starting the section with 75 RP. To make things even more puzzling, I then loaded up my save from just before leaving Zirantia and then started the Airship Negotiations again, and I was informed that she has 10 slots, as expected. Loading my first save and entering the Negotiations again consistently results in me getting 8.
I really don't know what I could have done to cause this discrepancy. In the save where I only got 1 extra slot I did go around and take care of pretty much all business in Yhilin before even entering the Airship Negotiation area, whereas on subsequent testing where I got the expected 3 I went straight there. But that shouldn't affect it if everything's working as expected as far as I can tell. Since I did get the one default extra slot, just not any of the bonus ones, it kinda feels like I somehow managed to bypass the check for Trin's RP. I'm not even entirely sure when that check occurs, The Calculator implies that it happens right at the start of Govern Yhilin III, but I'm not sure if that's 100% consistent with how the game actually handles it.
So yeah, I'm a bit stumped on what happened there. I tried loading up my save just before leaving Zirantia and retracing my steps a bit to try and narrow down a specific action that caused it, but I haven't been able to reproduce it outside of the one save where it always happens.