Actually I spoke too soon. I do wonder if it is something to do with having multiple types of GSV messages incoming. As it turns out we don't absolutely need GSV, but it's odd they aren't being processed:
tcpdump shows the GSV messages coming in, but in gpsmon I only see the GGA and the RMC messages
tcpdump dst 255.255.255.255 and port 6125 -A
$GPGSV,3,3,12,23,45,075,46,26,16,167,28,27,89,031,51,30,11,328,42,0*61................................
$GLGSV,3,1,10,66,10,006,32,67,30,056,34,68,21,110,32,73,33,211,43,0*77................................
$GLGSV,3,2,10,74,51,275,33,75,20,334,41,82,16,068,35,83,58,047,32,0*7E................................
$GLGSV,3,3,10,84,48,272,32,92,27,317,,0*75............................................................
$GAGSV,2,1,08,02,40,303,44,10,65,064,47,11,84,223,43,12,38,062,39,0*74................................
$GAGSV,2,2,08,19,12,067,25,24,19,168,26,25,59,220,46,36,32,239,38,0*7A
telnet localhost 2947
Connected to localhost
{"class":"VERSION","release":"3.19","rev":"3.19","proto_major":3,"proto_minor":14}
?WATCH={"enable":true,"nmea":true}
{"class":"DEVICES","devices":[{"class":"DEVICE","path":"udp://
255.255.255.255:6125","driver":"NMEA0183","activated":"2023-02-15T19:02:59.151Z","flags":1}]}
{"class":"WATCH","enable":true,"json":false,"nmea":true,"raw":0,"scaled":false,"timing":false,"split24":false,"pps":false}
$GPRMC,190300,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*17
$GPGGA,190300,5254.6985,N,00127.6425,W,1,12,0.5,52.9,M,47.5,M,,0000*60
$GPRMC,190301,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*16
$GPGGA,190301,5254.6985,N,00127.6425,W,1,12,0.5,52.9,M,47.5,M,,0000*61
$GPRMC,190302,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*15
$GPGGA,190302,5254.6985,N,00127.6425,W,1,12,0.5,52.9,M,47.5,M,,0000*62
$GPRMC,190303,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*14
$GPGGA,190303,5254.6985,N,00127.6425,W,1,12,0.5,52.9,M,47.5,M,,0000*63
$GPRMC,190304,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*13
$GPGGA,190304,5254.6985,N,00127.6425,W,1,12,0.5,52.9,M,47.5,M,,0000*64
$GPRMC,190305,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*12
$GPGGA,190305,5254.6985,N,00127.6425,W,1,12,0.5,52.9,M,47.5,M,,0000*65
$GPRMC,190306,A,5254.6985,N,00127.6425,W,0.01,305.32,150223,,,A,V*11