In Custom Villagers summoned with lots of trades, the scroll bar reaches the bottom way too early. It seems this is caused by a related issue (MC-147819) which was patched in 1.14.1, but it seems that this patch simply capped the scroll bar at the bottom rather than fixing the root problem. (That it just goes down too fast and too much)
The command I used is stored in lotsoftrades.txt (attached because it exceeds the character limit) for if you want to try.
Note: This appears to be a purely visual bug, as when scrolling halfway using the mouse cursor, the trade window scrolls halfway down as expected; only the scroll bar is in an incorrect position.
Confirmed to be still in both the Latest Release (1.15.2) and Latest Snapshot (20w19a)
I've also added a bit more information to help fix the bug.