Yes, unfortunately 8tb on one drive is not necessarily the exact same as 8tb on any other model drive, even from the same manufacturer. Where possible, you should replace with as close a model as you can get.

If you’re expecting to mix and match, it’s a good idea to leave a little slack space at the end to account for this. Else you may have to fiddle with the partitions as you found, or at worst start a whole new array with the smaller drive and migrate the others.