We're waiting for a RNP v0.17.1 release which intends to tolerate this mode.
https://github.com/rnpgp/rnp/issues/2198
https://github.com/rnpgp/rnp/pull/2190

In T5250#148131, @bernhard wrote:

BTW @kaie

Thunderbird cannot use anything requiring GPL in its default configuration, because Thunderbird wants to distribute a single MPL licensed package that includes all components that are required for OpenPGP.

Any pointer why, they have made that choice, though? A bundle of MPL and GNU GPL components is fully allowed by the licenses as far as I know.

I said "we're offering the optional use of GPGME

At the time I started to add an optional binding from Thunderbird to GPGME, I wasn't aware of gpgme-json.

In T5250#141705, @werner wrote:

Sure that TB uses GPGME - they claimed they won't use it due to license incompatibility (LGPL). I assumed they use gpgme-json via naticve messaging.