I have a collection (colTasks) One of the properties is es_week, which is a lookup (so a record itself). I can save this back to CDS without issue. I can modify the es_week for all colTasks with the following method and it will save to CDS without issue:
It updates colTasks as expected and other parts of the app reflect the change and behave appropriately.
However, on saving to CDS, PowerApps saves all the other records but does not even attempt to save the record whose week has been changed (I watched network traffic). Instead throws the error Value must be a data entity record.
On inspection of the colTasks.es_week using view collection, it appears to be property-per-property identical to all the other records, but something somewhere doesn't like it.
I reached out to Microsoft Customer Support through the Admin Center and they did a screenshare with me and even brought on a "senior engineer" who seems to think I've done everything correctly as well, so this might actually be a bug in the platform.