Bulk editing has the following options: leave as it is, replace, delete and append. So I wanted to add an asset owner to another one that I already set. Therefore I used the option “append” but instead of adding something it just replaced that one that I set before.
Yes i was able to reproduce this on r18: https://drive.google.com/file/d/1vdLob2sbbhkRJbtGf_BmhALQDoWne0eI/view
it would be interesting to test this on other sections and other field types (not user drop downs)