When you create a required single-value entity reference field, for example a term reference, then the referenced entity is deleted, the _none option disappears. When the widget is loaded up it defaults to the first option in the list, and the user can easily save the new value without noticing. Steps to reproduce below.
I believe this tracks back to OptionsSelectWidget::getEmptyLabel() which checks for !$this->hasValue. It doesn't consider whether the stored value is actually present in the available options.
Needs work
10.1 ✨
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I'm unclear why this was moved to Active instead of Needs work as the feedback in #44 is referring to the patch in #39.
So... moving to Needs work to incorporate feedback from #44 and #40.
@smustgrave You mention creating a followup in #42 but I don't see the issue noted as a related issue here. Please add when you get a chance.