Why does the minecraft launcher remove my jar file when I am not connected to the internet.

Hi,

I tried to make a mod for my 1.12 minecraft installation. To do this, I "patched" the 1.12 jar. However, it seems that as soon as I try to run this with the current launcher, it deletes the jar (even when not connected to a network). Why is this? I have the checkbox enabled to allow modified versions in my launcher too.

Any help?

Cheers,
Maarten

NOTE: The actual version I tried is 1.12, but it seems like I can't submit bugs for that.