Bingo.
That lead me to this thread - Dump1090 - Message rate too high in Skyaware - #32 by wiedehopf
Confirmed mode a/c messages with jq < /run/dump1090-fa/stats.json ‘.total.local’
disabled mode a/c in piaware-config.txt allow-modeac no
restarted dump1090-fa and rbfeeder, no more high message rate in skyaware/tar1090
For package install of dump1090-fa added EXTRA_OPTIONS=“–no-modeac-auto”