It's an over-aggressive Android memory management routine that's killing the SMS/MMS service (which then, eventually, restarts itself) and any messages that are delivered to the radio while the service isn't running are lost, as the radio has no "cache" to hold the messages until the service is available again to insert them into the database.
SMS/MMS delivery process:
Tower -> Radio chip -> Radio driver -> SMS service -> SMS database -> SMS app
The radio driver dumps any received SMS to the SMS service, but if the service isn't running at the time, the message gets lost. You can easily reproduce this issue on ANY ROM by loading an app that attempts to consume all
device RAM. Android will keep killing background processes until it starts killing some services. I can cause the same issue to occur on the stock
Telus ROM by trying to launch Draw Something (which runs) or another memory-hungry app.
It's not a bug in this (or any other) specific ROM, it's an issue with the Android memory management system that can be reproduced on any device (although it's much harder to reproduce on devices with more RAM) and any Android version.