Sometimes with screen protectors,,especially matte variety, the pencil tip CAN get loose from use.
Some glass screen protectors can be too thick for the Pencil to work properly.
Thicker glass screen protectors are NOT always desirable with the use of an Pencil because it puts the Pencil tip and sensor too far away from screen and the underlying layer responsible for an Pencil to work.
I would try your Pencil without your current glass screen protector and either opt for a thinner glass screen protector or a thin, plastic film screen protector instead. Either the smooth clear variety or a matte film screen protector, when having to use an Pencil.
Let me know if removing your glass screen protector off your iPad has solved the wavy diagonal line drawing issue with your Pencil.
If you need some Pencil drag on the tip of the Pencil, there are makers of silicone, flexible tips you can put over the Pencil tip so the Pencil is less slippery when writing/drawing on the glass surface.
Let me know the end result of removing the glass screen protector, either way.
Best of Luck to You!