Along with or near to the RGB pins on the jungle chip, there should be a pin for fast blanking. In this case it is pin 38, labelled "FB". This pin needs to be fed from +3 to +5VDC to blank other signals and show RGB. At the moment, it looks like this pin is connected to the original OSD FB line from the set's microcontroller chip. This blanks the display at specific points and pulses RGB in, to display the text of the menus. But instead of the menus, you're seeing your RGB signal. So the RGB lines are connected, but not the FB.
It *might* be more complicated than this, it's possible someone has muxed the OSD badly, or a wire has come loose. There are jungle chips that blank outputs on higher voltages and other things. If you get stuck, there's a dedicated RGB hacking thread on the Shmups forum -
https://shmups.system11.org/viewtopic.php?f=6&t=56155Most important of all, remember to be safe around live equipment, and that even powered-down a CRT might hold dangerous charge for days.