With TOAD 16.3 Beta 16.3.230.2084
When I modify the size of a datafile (Auto Extend On) of a tablespace, after OK, the "Maximum size" became UNLIMITED.
Thank you.
I see what's happening here.
See how the max size units dropdown is disabled in my screen shot below? That's when it happens.
If you manually re-type the max size (3 in my case), then the max size units will become enabled, and then it will work as expected.
You shouldn't have to do that, of course. This is a bug and I will fix it for the next version.