Forms with drop-down menus or areas that need to be filled out flash and then do not retain the selection. Works fine on a PC but not on the lastest version of Mac or prior version of Mac. I have had this issue for a long time but no solution yet to solve it. Could be a java script issue.
Are you filling the form with Preview on Mac? What app name is shown on the menu bar as you are filling it?
Forms do not work well in Acrobat DC Reader. Once I upgraded to Pro the issue went away.