The 7.59 update seems to have changed the way mIRC handles "if (0 isnum 1-)". Before it would result in false, but now it's true.
I assume this is not intended.
Thanks for your bug report. This issue has been fixed for the next version.
This will need more testing in the beta to make sure that the change to make it support both small-large and large-small ranges is working correctly.
The latest beta includes a fix to resolve this issue. As far as I can tell, isnum is now working as before, however if you spot any issues with it please let me know.
It appears to be working as expected now.