Thank you for this interesting question.

This is actually the intended behaviour of the monophonic couplers. In short, we avoid 'Note Off' events (i.e. a key released) which trigger 'Note On' events (i.e. a key pressed) on monophonic couplers. These are generally unwanted since they produce notes that are actually not played by the organist in real-time, except for 'fast' legato style - which is automatically detected by the monophonic couplers. You can try to play fast ascending and overlapping notes on the Great while the 'Bass' mono coupler is on to see that.

To go back to your example, this is what happens sequentially here :

- C48 is pressed on the Great.    ->   C48 is sent to mono. coupler Bass
- G55 is pressed on the Great.    ->   C48 is still on m.c. Bass
- C48 is released on the Great    ->   C48 is released from the m.c. Bass. No jump to the G55 for the m.c. Bass, as it is considered musically not relevant by the monocoupler algorithm.

As the G55 is still pressed on, it will not be sent to the m.c. bass until the G55 is released then pressed again.
Any following note played on the Great might be sent to the m.c. Bass.

To reproduce, follow the steps (see video description) in this short video

As it's entirely possible this is just me not understanding how organs are supposed to work, feel free to let me know...