Sorry, perhaps the support is just trying to figure this out on their own - you should have told them you've already escalated the case into R&D through me.
Devs reviewed your logs and we did find the missing piece. Everything is OK, and the reason is actually much simpler than we thought. It appears that shortly after you've done migrating your backups to ReFS volume, compact operation was triggered by the schedule on some jobs, which created the new VBK that full meets the requirements for fast cloning. And considering that with reverse incremental backup, only VBK and the latest increment are participating in merge, fast cloning immediately started working for you.
So, feel free to close the support case since all is well in your case.