It's not just acid migration you gotta be worried about when using these kinds of sheet goods as a backing, but potentially also formaldehyde
in the glue, or other outgassing nasties. Leave masonite for lining doghouse floors.
There isn't any formaldehyde in masonite. It is simply hot pressed steamed wood fiber held together by its own lignin. The process is simple but requires high pressure steam and high pressure rollers. MDF and other processed wood panels do contain formaldehyde, as they are ground wood and adhesive.