When saving a pre-existing non-default revision, for example a revision to a draft, the entity system would previously compare field values against the default revision to see if database tables need to be updated.
This caused a bug when field values were changed to match the values in the default revision, because even though they had changed in the draft revision, the comparison to the default revision showed them as unchanged.
Instead, the previous version of the draft revision will be compared against, so that any changed values are actively updated.
This should have no effect for most sites, however custom code or modules that are working around this bug will be able to remove those workaround once they require Drupal 10.1.0