I've repeatedly encountered an odd problem editing items with multiple contributors. When I delete one using the (-) button, that line visually disappears but it is replaced with the one below(?) it, so the wrong person is deleted. Similarly, if I clear a name manually just by removing the text in both first and last name fields at the top of my list of contributors, it deletes that line but then also one below it!

Zotero 5.0.90 / OSX Mojave

(My impression is that this may be new since 5.0.90, but I'm not certain. It seems to be consistent and easy to replicate.)