Well.
I just tested my GD77, and it can receive on CTCSS simplex, from when the radio is in full power saving Level 4
I had to hold the PTT for almost second before the signal is detected, but this latency is documented in this thread.
I'm now waiting for a local repeater which uses CTCSS to go idle and then be activated while the radio is in full power saving, but so far I can't replicate this problem despite now wasting 30 minutes on this due to the lack of useful information from you.
I'd appreciate it if anyone else could test CTCSS reception, but so far I see no sign of a bug.