You may be getting bitten by an old cache file. If this system was upgraded from FreeBSD 12.x to 13.x, there may be an old cache file in /boot/zfs/zpool.cache that still has the old name.
You can just delete the copy in /boot/zfs and it should solve the problem.
If that doesn’t work, I’d suggest using zpool reguid newname after having done the rename. This will assign the pool a new GUID and rewrite all of its labels. This will invalidate any wrong cache that might be persisting from somewhere you don’t realize.