This happened several times when a faster Pokemon broke Ice Face and Eiscue used Belly Drum or Substitute on the same turn. Belly Drum brought Eiscue down to 0 HP but didn't faint it, instead causing the battle to be stuck on waiting. Substitute took 50% HP and the battle proceeded as normal. If Eiscue used an attacking move on the turn Ice Face was broken and then used Substitute/Belly Drum on the next turn, they would take away the normal amount of HP.

In addition: using /endbattle when Eiscue hits 0 HP (and the battle freezes) leaves Eiscue with 50% HP as expected. Similarly, using /endbattle after Eiscue takes 50% from substitute leaves him with 75%. But it seems like the battle still progresses as expected when eiscue is at 50% from substitute if you don't endbattle. So it might not be entirely visual? Not sure.

Apparently Z-Belly Drum is also restoring 50% HP for Eiscue (haven't tested on anything else). I'm gonna leave it here because I think you get the idea lol.

Thank you for posting, I was able to replicate the bug

Thanks for the report. I've now moved this over to our internal tracker, and will have this ticket's status updated if anything changes.