This is not the normal behavior.  Does this happen only with one
presentation or one template?  Try creating a new presentation based on a
blank slide.  If that keeps the formatting, you should create a new template
based on the blank template. You may copy any images from the old template
to the new one, but you should not copy text formatting.

Best regards,
Ute

Thanks for including your version of PPT!

There's a bizarre bug where PPT reverts to the first master in the list of
slide masters.

You can try moving your master with centered text so it's first in the list.
But that solution never feels like a solution to me because you run the risk
of affecting other slides.

So what I'd do is copy the text placeholders and paste them back onto the
slide, deleting the original placeholders. The copies are no longer
placeholders and so they're no longer tied to the slide master. And so they
*should* stay centered.