Thanks for the file.
I took a look and was able to reproduce the problem.
Somehow, the checkbox has some sort of a script attached to it – maybe at one point it was a text frame?
Anyway, whatever the case, try selecting the checkbox in InDesign. Change it to a “text field” and then back again to a “checkbox”. This will clear out any old scripts that were lingering. Make sure that its name is still “Check1” after you change it back.
Now export the interactive PDF with FormMaker, open in Acrobat, run FormMaker, and everything should work as expected.
Let me know if that fixes the issue for you!