I do not see any difference between opening in v 12 or 13.
I'm not sure why this is happening but it is certainly not behaving.
I suggest the only option you might have is to either choose a theme to apply to the layout or select the fields and clear all formatting... so you can then apply your customisations.
In both 12 and 13 there are aspects to a theme (CSS now) which you cannot control within FMP.
Best of luck
Was this file converted up from .fp7 format? The reason I ask is I've seen similar behavior on one of my solutions, and even stripping out all the non-theme styles didn't seem to help. I've also noticed that, when input from an iPad, the field looks normal ... until you look at it on the Mac. Then it looks weird, until you click in the field and back out, when it goes back to normal.
There's definitely something strange going on with the rendering, though.
The file was not converted up from .fp7.