Are you sure the original drive was mountable before you replaced it?
If the original disk was unmountable, then if parity and all other disks were in sync, the rebuild would have resulted in the same unmountable contents.
Or, if parity and all other disks were not in sync, then rebuild could become corrupt and unmountable.
If we still had the original and it was mountable, then we would know something else was the cause of the corruption.
Do you do regular parity checks? Do they always result in exactly zero sync errors (the only acceptable result for accurately doing rebuilds).
Were there any errors shown in the Errors column during rebuild? Do any of your disks display SMART warnings on the Dashboard page?
When rebuilding a disk, what you should see is a lot of writes to the rebuilding disk, and a lot of reads of all other array disks including parity. And zero in the Errors column for all disks.