Perfect forward secrecy is actually the opposite of what you mentioned.
"In cryptography, forward secrecy is a property of secure communication protocols in which compromise of long-term keys does not compromise past session keys."
https://www.wikiwand.com/en/Forward_secrecy
If someone were to compromise your key and they had a packet log of all your communication then PFS, which Signal has, guarantees that they wouldn't be able to derive previous keys from the current key to decrypt previous messages from the packet log that came before the key compromise.
The thing you're talking about can be resolved by revoking compromised keys but knowing when to revoke those keys is a whole other problem that hasn't been solved by anyone to my knowledge...
"In cryptography, forward secrecy is a property of secure communication protocols in which compromise of long-term keys does not compromise past session keys."
https://www.wikiwand.com/en/Forward_secrecy
If someone were to compromise your key and they had a packet log of all your communication then PFS, which Signal has, guarantees that they wouldn't be able to derive previous keys from the current key to decrypt previous messages from the packet log that came before the key compromise.
The thing you're talking about can be resolved by revoking compromised keys but knowing when to revoke those keys is a whole other problem that hasn't been solved by anyone to my knowledge...