brainsick

That particular backup is not viable.  Can confirm that it's 0 bytes so there's nothing to import.

Is this possibly a chunk border issue as described in MCPE-152014?

I can confirm we've experienced this behavior.  Transported two living shulkers from the overworld -> nether -> overworld again.  When we got the first shulker to the overworld farm location and broke the minecart it disappeared in front of our eyes.  We eventually found it back in the nether.  I have video of the disappearing act from July 15th, which indicates we were on v1.20.10.

When you reloaded were you at the same location/coordinates as when you froze?  Was your experience reset or did you retain your levels? 

It's possible that you were attacked while frozen and died.

We're experiencing this issue frequently.  Bedrock Dedicated Server 1.20.10.1 hosted by Shockbyte.  For us, it happens when going from the Nether to Overworld.  Using established portals; meaning the game is loading already generated chunks.