It seems to be “newer” clients. nostr:npub1zafcms4xya5ap9zr7xxr0jlrtrattwlesytn2s42030lzu0dwlzqpd26k5 is it because a specific NIP is missing or any general ideas.
Discussion
Op mentioned flockstr. Its difficult to know without the full multi app history.
This is where something like #nostrCI would come in - automate this type of testing, and reporting.
Otherwise its manual sleuthing using incomplete info