So I went ahead and checked, 78.1.1 was the last version to reliably decrypt subjects.
I took every version since 78.0, cleared the Thunderbird folder each time. After which I simply enabled OpenPGP support (for <78.2), imported my private key, imported an email I had saved, restarted Thunderbird and tried to read the imported email.
78.0 through 78.1.1 worked without issue (albeit I had to restart Thunderbird after importing the private key).
Surprisingly, 78.2 through 78.3 worked before restarting Thunderbird. But once I restarted Thunderbird, I either couldn't decrypt subjects or the entire OpenPGP support would silently fail and simply state that OpenPGP support was no available (despite being enabled when checking about:config).
Is there a MOZ_LOG options that can be enabled to debug the OpenPGP component?
I tried it multiple times, the issue seems to be fairly reliably reproducible (at least on my computer, might be some other setting interfering).