Just going through this now which led me to this thread.
Unraid 6.11.5, just swapped my parity drive from 6tb to 18tb.
When I booted up, I added the new parity drive to the empty slot and started the array. I started to search for something in another tab and the Unraid tab stole focus when the alert box popped up and I hit enter at the exact same time so it accidentally did the "Resend" option.
Now I'm stuck with "Array Stopped * Parity-Sync 0.9% * stale configuration" at the bottom, but all Dockers and VMs started up and everything seems to be working even though the GUI doesn't agree.
Guess I just have to wait it out now for a couple of days until this process finishes.
I think everything is OK, but it's very unnerving when already doing such a big operation.
Guess I should have updated the OS before I did this, but I've been otherwise very happy on 6.11.5 and didn't want to add other unknowns to the mix.