I have a instances of signal-cli running as signal-cli -a +xxxxxxxxxxx daemon --socket --receive-mode=manual. It is driven by a simple script with "maxMessages":1. The script is here: https://github.com/etwasch/signal-forwarder-bot
signal-cli is set up as primary device, doesn't have linked devices, and is a member of a low traffic group (5-10 messages a day).

Very rarely it skips/misses incoming messages. There is no trace of them in the console output of signal-cli and there are no error messages.
I can't reproduce it, when sending a 1000 messages to a test group, it didn't happen.
I suspect that it has to do with --receive-mode=manual and "maxMessages":1.
Another instance of signal-cli on another account that runs as signal-cli -a +yyyyyyyyyyy jsonRpc never skips a message.

Has somebody an idea what's the cause of this or how to debug/narrow it down?
How does signal-cli running as --receive-mode=manual and "maxMessages":1 work? Does signal servers support receiving one message at a time or is there a cache file on disk?

Any help is very much appreciated.