I was curious so I used git-bisect to find out when this was fixed.
It was fixed by
Commit: 8e7ddc5f50af ("Grow: fix problem with --grow --continue")
in mdadm 3.3.3
We like to keep Leap42.1 the same as SLE12-SP1, so it might be good to backport
this patch to SLE12-SP1 and then submit that to Leap42.1. Maybe.