Strava doesn’t give out email via their API. They used to when I started Intervals.icu bit not for at least a year now. So changing your email on Strava won’t impact Intervals.icu.
Aha. You need to change it in /settings in Intervals.icu. You might need to logout of the forum and re-login after doing that to get it to update in the forums.
I have a similar issue. Having changed email in strava, I sign into interval with strava but its not updated. I cannot change the password in intervals.icu in the forum or settings.
It says ;primary
Email can be updated from authentication provider. Never shown to the public.
Your email on Intervals.icu is separate to whatever is on Strava. You need to login and then change it on Intervals.icu. Then logout of the forums and login again and it should update.