During the processing of the NewProvider(context, string) function a comparison is made between the issuerUrl passed into the function against the issuer URL retrieved from the /.well-known/openid-configuration (line 235 in current main code). Earlier, when the wellKnown variable is created (line 209), a trailing / in the string is ignored when concatenating the URL information together. However, when making the comparison on line 235, a trailing / is not ignored.

Ex:
issuerUrl = "https://www.myoidcprovider.com/"
Provider returned issuer = "https://www.myoidcprovider.com"

outcome - The below error message
oidc: issuer did not match the issuer returned by provider, expected "https://www.myoidcprovider.com/" got "https://www.myoidcprovider.com"