This seems to be the problem reported in r#31768

I can reproduce this e.g. when trying to decrypt z:/15_ThisIsAveryLongFolderName_1/15_ThisIsAveryLongFolderName/14_ThisIsAveryLongFolderName/13_ThisIsAveryLongFolderName/12_ThisIsAveryLongFolderName/11_ThisIsAveryLongFolderName/10_ThisIsAveryLongFolderName/9_ThisIsAveryLongFolderName/15_ThisIsAveryLongFolderName_1.tar.gpg

Decrypting works fine when starting it using the Decrypt/Verify action in kleopatra, but fails when starting it by opening the encrypted archive in the windows file explorer.
In the broken case (when starting it from the windows file explorer), the path passed to kleopatra starts with \\?\ (see windows docs). In the working case, the path does not have this. This seems to be the relevant difference; if i remove the \\?\ from the path in kleopatra, decryption starts working fine. (I don't know whether that is a useful fix, though).

GpgME also breaks the \\?\, since it replaces all backslashes with forward slashes on windows, but fixing that doesn't seem to make decryption work, so there has to be something else going wrong somewhere