When selecting multiple items and adjusting the fade curve via mouse modifier, only 1 item's curve is actually adjusted:
Here is a thread discussing the issue:
https://forums.cockos.com/showthread.php?t=230876
Some people (like post #8) say that adjusting fade curve works, if fades are of the exact same length. On my Reaper that is not the case, it just never works.
Anyway there is definitely something off here and it is cumbersome to work like this. Adjusting fade curve with the mouse should work on all selected items and also with various fade lengths.