Actually, this should not break connection persistence at all because the realm of the connection is picked up then immediately saved in the mangle POSTROUTING chain.
I probably does, however, break QoS. (I’m guessing that the actually device queuing happens after the POSTROUTING chain and at that point the mark is set based on the routing realm.)
Can the QoS tagging be moved to the POSTROUTING chain and be done after the realm tag is saved in the connection?