This is a known issue with Doom 1 and will be fixed in the next beta.

Open the CBI menu and click "Inventory."

Update GZDoom.

If you shoot the light source until it is destroyed, it will no longer be usable. You can do that to fix the issue.

You probably need to update GZDoom. It will only work in 4.3.3.

You need to use GZDoom for now. LZDoom should be able to run it whenever it gets the new font system.

Issue has been fixed as of 1.6.2.