Deleting files doesn't clear the drive. The entire drive must have zeroes written to it, so when it is removed the parity is still valid. Preclear DOES write all zeroes, but you can't preclear a drive while it's in the array. There is a script that does clear the drive while it's in the array so you can remove it without recalculating parity, but the script doesn't work well for many people, it's very slow.
I think you need to stop the array, unassign the parity drive, start the array so it forgets it, stop the array, assign the parity drive so it will recalculate from scratch.
A clear drive in Unraid terms is all zeroes. A drive with all the files deleted, or formatted, even if it shows an empty filesystem still has a whole mess of ones and zeroes on it, some from the deleted files, some from the filesystem itself.