As replied on the fediverse, your QuoteAuthorization has an id that points to a different activity, so it cannot be verified and is treated as invalid.
Mastodon.social got an Accept from your bot server so trusts the stamp is valid without checking the stamp. This is a choice we made to cut down on traffic and simplify implementation, trusting that the authoritative party will create a valid stamp accompanying the Accept. This has the downside of leading to inconsistent results when there is an implementation bug, though.