Unraid does not automatically format a drive. If the drive came up as unmountable either prior to or after the rebuild process, a popup would have showed up that stated that "Formatting is never part of a rebuild process". Did you tell Unraid to format the drive?
This is what caused the problem. By changing the format of the drive, it basically would've had to format the drive. (But still would have prompted you)
What you want to do is take a note of all the current drive assignments, do a Tools - New Config, and then reassign the drives and substitute the original data drive back in in lieu of the replacement. If anything comes up as unmountable, STOP and then post your diagnostics and wait for a response.
Let parity rebuild and then do the substitution.
After that, to change the format of the drive, you first have to copy all of the files off of that drive and then once it's empty format it.