I'm trying to think of the name of the function, so play along and let me call it:
Show Toolbars(On) or ShowToolbars(Hide)
Methinks that you have the hide version nestled in a script somewhere... Probably since you developed in Pro and don't want to see the toolbars.
You can do this
If(get(systemversion) = 3)
Now where to put this?
You can start every script with this or layout trigger etc. But I have a feeling that you will have to trace down the culprit by stepping through your solution and finding those places where the tool bar disappears, opening Filemaker Pro and checking out the scripts that go you there. Have fun!
Personally I have ONE script that is called for a new layout or window and so I have only ONE script to debug...