Based on this PR, when Cosign was upgraded from v2.5.3 to v2.6.0 (PR #1441) it introduced a breaking change in the algorithms supported for identity tokens. That PR specifies in the release notes that there was a breaking change and documents the action required:
Action required:
Update your identity token generation to use RS256 algorithm. If using custom token generation scripts, ensure they sign with RSA keys
Neither the cosign upgrade nor the breaking change it's required-action is mentioned in the v0.26.0 release notes