Looking at the stats collected by FHODOT, lots of FHRS IDs changed on 7th March as per the FSA announcement, and were gradually corrected until 21st March. Then on 28th March a lot of OSM objects (c. 300) went from being matched with the same postcode to matched with different postcodes.

I'm afraid I can't shed any more light on what could have caused this final change but it looks like the FHRS IDs were shuffled around rather than new, previously unused IDs being assigned, which seems odd. Hopefully the FSA might be able to help.