@mpa : You are right – I did a key import of the new private key exported from DeltaChat in gpg and it looks like, that the public key is always part of the secret one (or is derivable from it) because both are now present in gpg. So it is unlikely to be the problem…
But gpg is the much major application than DeltaChat and I do not think it does anything „harmful“ to the key on import and export. It shouldn’t matter at all if the key is slightly different after passing through gpg, because Deltachat should accept existing keys from gpg and others:
FAQ: Delta Chat supports common OpenPGP private key formats, however, it is unlikely that private keys from all sources will be fully supported. This is not the main goal of Delta Chat. In fact, the majority of new users will not have any key prior to using Delta Chat. We do, however, try to support private keys from as many sources as possible.
If an ordinary import/export breaks compatibility it makes no sense to claim “supports common OpenPGP private key formats” or “…try to support private keys from as many sources as possible.” And according to the info in this link (the link is from DeltaChat FAQ!): https://github.com/deltachat/deltachat-android/issues/98#issuecomment-378383429 the import/export (and deleting an existing password protection) in gpg is recommended and was at least working in 2018.