Using custom story size causes paragraph formatting on slide masters to change

Hi!

I have been building courses in SL360 with a custom story size of 900x600 for several months, and I make heavy use of slide masters.

But now the paragraph formatting on slide masters changes when I change to a custom story size. It took me a while to figure out that the custom size was the cause of the problem. If I use either of the two built-in sizes, there are no problems.  And it doesn't seem to affect the paragraph formatting done directly on the slides, only the masters.

I'm trying to find any kind of work around, with no success. The formatting never stays where I put it, regardless of when I change the story size. I've tested this on existing projects and new blank projects, same results

Any suggestions? Here is a Peek video of my process, and attached is the sample file. 

https://360.articulate.com/review/content/4266c1c6-0a11-43e9-9b4c-945980fccb03/review

Thanks!

(I've turned this in a support issue, but they said it would be a few days before they could get to it, and I'm starting to sweat!)

UPDATE: I've been advised by customer support that the issue may be a bug, and is tied to the specific dimensions I'm using (900x600). I've adjusted my size to 896x540 , which solves my immediate problem with a new course. Hopefully the bug will be fixed so I don't have to adjust any previous courses. :)