If the entire supply chain up to the software you're running to perform actual decryption is compromised, then the decrypted data is vulnerable. I mean, yeah? That's why we use open-source clients and check builds/use builds from separate source, so that the compromission of one actor does not compromise the whole chain. Server (if any) is managed by one entity and only manage access control + encrypted data, client from separate trusted source manage decryption, and the general safety of your whole system remain your responsibility.

Security requires a modicum of awareness and implication from the users, always. The only news here is that people apparently never consider supply chain attacks up until now?