Rotating backup (send-only) frequently out of sync

I use Neo Backup (formerly OAndBackupX) to make backups of my phone. It creates backups in format “date_filename” and keeps 7 days rotating, e.g. as of today (November 18), backups from November 10 are deleted on the phone.

In Syncthing the phone (v1.22.2) is set to send-only and the server (v1.22.1) to receive-only. This has worked as expected for many months without any interaction.

Recently the server is often out of sync - but only on the files that were deleted on the phone (e.g. from November 10). A click on “revert local changes” solves this.

I see that there is a bug with permissions in v1.22.
Please advise if this is also the cause here, thanks!