Cutoff vs emissions

[Notimetolooz]

CRT testers operate the tube like a triode. The emission reading comes from the current between the cathode and the first anode. The cutoff is the grid bias needed to cut off the triode.
If there is gas in the CRT, then the current is partially due to ionization of the gas and that is not controllable by the grid. So this may be what is happening.

Can you see the getter flash?
If it is turning whitish then that would confirm this.