I think avast is not yet ready with its new firewall architecture.
Me too, it doesn't want to retain some rules when it is configured on "ask" (adobe acrobat , avast secure browser , avast browser update,...).
This had been solved on version 20.2 by an emergency update, but since the upgrade to version 20.3, the problem has come back as well if you allow or deny some applications (it also happened once with thunderbird)...I think there are still some failures in the build of the firewall.
A repair has no effect.
Of course it may be possible to solve the problem with a clean re-install or by sending a support file but I'm a bit tired of paying for a product that takes so much effort to be usable in full reliability and openness.
edit :2 new times for thunderbird.