Hi @umxm and welcome to the community.
Are you using Operator v0.9.0 and above? Upstream mTLs was introduced from that point.
If yes, then can you check your gateway security configuration for the value of private_certificate_encoding_secret?
The thing was that certs used for upstream have private keys. Certs with private keys get encrypted
The encryption key is either the setting in private_certificate_encoding_secret or the gateway secret if private_certificate_encoding_secret is not defined
As an addition, can you confirm if your hashing algorithm may have changed recently?