RPMSG has no limitations on characters used. Check your TTY settings, perhaps TTY layer is not in RAW mode.
RPMSG is working well on iMX7. However look at @hfranco.tx mentioned excerpt from kernel docs in recent thread here. Once your A-side TX buffers are exhausted, RPMSG will fall asleep for 15 seconds… I guess you see just that, not complete death of RPMSG but this 15 seconds pause.
How to not unexpectedly waste your buffers and so not face this 15 seconds pause. Having a lot of data to send, don’t sent a byte or few bytes at a time. Instead send as much as you can up to 496 bytes at a time in one write() call. 496 comes from max buffer size 512 minus 16 bytes for RPMSG header. Both sides, Cortex-A and -M should follow this rule. One transfer triggers one interrupt. High interrupt rate is performance killer for every CPU. Sending by single byte instead of burst of bytes you multiply interrupt events. Interrupt rate is not so big problem for Cortex-M, but there are delivery to CM interrupts which are sent back to CA.