This is a known issue to CSI, but apparently we are awaiting a fix from the manufacturer.
This will happen not only in monitor mode, but also with programmed channels as well.
Thing is, it doesn't happen right away. The problem is exacerbated sometimes by long-winded nets / qso's with a very delayed carrier drops, and if/when the drop finally does come, audio is restored. In the meantime, you could miss many minutes of comms.
The short-term solution when this happens is to temporarily move the channel knob away and back. Presto - audio restored. For monitor mode, you'll have to toggle it.
I too have other radios, that do not behave this way when sitting side by side, so this audio lockup was really frustrating, including the earlier 700 which does not exhibit this behavior.
Those who only monitor short transmissions, or do not have any other radio to compare it with, may never notice, but attribute the problem to the system itself, when the firmware in the 750 is what needs updating.
I also mentioned the fact that the talkgroup should not remain active when you unscrew the antenna (emulating loss of rf) when the tg has active comms. No other dmr radio I know of hangs on to the talkgroup when there is a major loss of rf. In fact, depending on how weirdly you unscrew the duck for this test, you *may* initiate the audio lockup - requiring a knob twiddle away and back as the temporary fix.
Might be worth mentioning to them to help convince the manufacturer that this is indeed an issue, and not just a bunch of guys unfamiliar with dmr. I have - perhaps others who can demonstrate this like you did with your 6550 sitting side by side might convince them to fix the firmware issue - their earlier version -the 700- did NOT act this way.