It can't validate via the signature, it can only trust that the signature has been applied to a fully cleared disk. The signature is placed in a logical location on the disk such that any write access that would normally be applied to a fully blank disk will destroy the signature.
If you were to take a precleared disk with signature intact and manually flip bits NOT in the signature area, Unraid would still trust it as precleared, and subsequent parity checks would find the altered area and either only alert or alert and alter the parity disk to match, depending on whether the check was correcting or not.
If there was a disk failure before those bits were corrected in parity, then each altered bit would be incorrect on the emulated disk, causing anything from file system corruption to absolutely no noticeable effect if the bits were in unoccupied space on the emulated drive.
Clear as mud?
At some point between the finalization of the preclear process and adding the disk to the array, the signature was overwritten, causing Unraid to require a clearing cycle. When that occurred can be very tough to pin down, but any attempt to mount or read the precleared drive could possibly cause it.