you reference PGP, Thunderbird does not use PGP as Linux users understand it, it maintains it's own key database. See https://support.mozilla.org/kb/introduction-to-e2e-encryption#w_how-to-use-openpgp-end-to-end-encryption-with-thunderbird

Sorry for the potentially bad wording. I'm using the integrated Thunderbird key database/encryption feature. However, it seems the UI for Linux differs from the one in MacOS such as I'm not able to decrypt messages in Thunderbird for Linux. See the screenshots above for reference.

Plenty of MUAs (such as Thunderbird) don’t handle inline pgp when it’s inside a HTML document, which is what happens when you hand paste a PGP message block into most webmail systems. That’s why people keep saying “I couldn’t read your email until I hit reply”, because if you have replies set to plain text by default it renders the plaintext version of the original mail in the compose window, which is then correctly parsed - but not before. So tell the sender of the troubled encrypted messages to use PGP/MIME. Why this works on the Mac I have no idea.

I'm afraid that's not too helpful. In this case, the sender is a corporate user who doesn't have much influence on what their mail backend is doing in regards of e-mail encryption. From my perspective the point here is actually why TB Linux cannot handle it the way TB MacOS does. - In fact, TB Windows is working even better by simply showing the decrypted message, instead of asking the user to press a button...

It shouldn't be too hard to send inline PGP encrypted messages in plain text, instead of HTML.