Problem/Motivation
\Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase::getOptions() computes the options for the widgets extending from that class and as it offers an alter hook it makes it possible to have dynamic options, which might change during a form rebuild. But as the options are set the hook will not be called anymore and not allow changing the options.
Proposed resolution
Don't cache the options or find a way to reset them before the form is being rebuild.
Remaining tasks
Add tests
Review
Commit
