When you have an entity_autocomplete form element (not talking about field widget here) with '#tags' => FALSE, then it should only accept a single value. There are actually two issues here, but I'm combining them because they're so closely related:
Problem
#tags were TRUE. So if you type in a comma and another letter, it's going to autocomplete that, which suggests to the user that you can enter multiple items.Proposed solution
Needs work
9.5
Last updated
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The patch in #25 does not work because of a malformed RegEx. My patch uses Tags::explode instead.
+1 Just stumbled into this ... IMHO it makes more sense to prevent completing multiple values client side in the first place. Just adding serverside validation is good but will just raise questionmarks in the opposite direction.