I wouldn't see that problem it for two possible reasons... I don't access any data via serial ports and our data rate is so high there's almost never a gap of more than a couple of hundred milliseconds between messages (more typically a millisecond or two).
I am chasing a different AIS problem that seems to have popped up recently. In fact I didn't see it until you made me aware that I was mis-configuring gpspipe for split24. When accessing gpsd via 'gpspipe -2' there are times when a single AIS messages (always a type 24) is repeated many thousands of times.
I haven't made a bug report because there is still some work for me to do to be absolutely sure it isn't a problem of my creation in the programs that accept the packets from providers and forward them to gpsd via udp.
I'll let you know if I eliminate my programs and I'll try to see if I can set up a test harness set of programs that will repeatably cause the problem.