To put this in more general terms, when there's some punctuation at the end of someone being mentioned, we only consider that punctuation if it's used in a somewhat-punctuation-related context. If you type "a.b.", that would be considered attempting to tag "a.b" (not "a"). The issue here is that when "[" followed punctuation, we weren't taking the punctuation to be the end. Demo:
@Mike[
vs
@mike.[
Fixed now though.