The serial port redirection often gets stuck in some kind of echo loop with devices, making it completely useless. This can be seen with the simple test command:
Disabling echo in the remote side (e.g. the modem) avoids the issue.
The bug appears often, but not always. Not sure what the conditions are.
The client doesn't seem to matter as I've seen this on Linux and Windows, and I've seen it both on "real" ports and USB ones.