I noticed the same data loss to Flightaware after disabling ipv6 on my R-Pi.
Locally, everything was running fine, but I later got an email from Flightaware that I wasn’t feeding?!
Thank goodness for the email otherwise I might have lost my 7 year streak.
It’s simple to replicate the ‘problem’ - just add “ipv6.disable=1” to the end of /boot/cmdline.txt and reboot.
Good to see Flightaware are on top of it and will have a fix in due course.
Nigel.