EntityReferenceItem::preSave() saves the entity stored in $this->entity if it is new and unsaved, itself. This is because the field must know the ID of the referenced entity, in order to well, reference it.
This is problematic when saving an entity which contains a computed reference field inheriting from EntityReferenceItem and the contained entity is not intended to be saved, e.g. when creating sample content via ContentEntityStorageInterface::createWithSampleValues().
Consult the field definition and check if the field is computed, before saving.
Needs work
10.1 ✨
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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Can the MR be updated to 10.1 please
Also will need a test case showing the issue
Thanks!