I just got an item labelled "Bunny Costume" as loot on a quest, but the item description appears to be bugged, as it constantly changes to be the same as the last item I looked at.

Yes the description code references a non- existant modifier. You can correct this by: 

1. Find the data\items.gd  file in your strive game folder. Make a copy.

2. Open the original in a text editor  

3. Search for service_suit, 

4. Then find the line that starts with 'basestats' line 1430 if your text editor show line number

5. Find mod_service in that line. change it to mod_pros 

6. Save the file.

This should fix future bunny suit items, but I don't think it will fix the one you already have.

Sorry for the line spacing. I can't seem to fix that.