@Phaedrux Thanks for the reply.
I thought I had deleted the M505 command off of that SD card, but I hadn't, so I tested without that and that was the cause of the M98 issues and the missing heightmap.csv file, so that mystery is solved.
I still don't know where it was getting x=25 as the starting X position on the old card. I'll go back make sure I've got the paths right on that card.