I have set up SP in wso2 Identity server and was able to get access_token in auth2 using playground. Ref : https://docs.wso2.com/display/IS510/OAuth+2.0+with+WSO2+Playgroun
The issue you are facing seems to be reported in [1]. Issue is originally reported for IS 5.1.0 and there were some inconsistencies in 5.2.0 release. According to the jira this will be fixed in 5.3.0 release, which is scheduled to release soon.
[1] https://wso2.org/jira/browse/IDENTITY-4250
It seems the logic to return claims by UserInfo Endpoint for scope openid is only the intersection of claims configure at OIDC file and claims configured in SP level are returned. Based on this logic I was able to solve this problem by following steps.
Look at values of Claim Uri and Mapped Attribute of the email claim at http://wso2.org/oidc/claim dialect
Now for the request
curl -k -H "Authorization: Bearer 8b2805a7-5b74-33ee-84df-2a8c53a16f6c" https://localhost:9443/oauth2/userinfo?schema=openid
you would get response
{"sub":"admin@carbon.super","email":"admin@wso2.com"}