I am also seeing this after using the newly released account purchases migration.
In my case:
I have a work email for ASC. That email is part of an internal team
I have an old "purchases only" account that I was signed into TestFlight as and was able to use the internal team TestFlight builds fine.
I have a personal primary Apple account that I am signed in for iCloud etc.
The steps I went through:
removed all apps from TestFlight on my purchases account.
did the purchases migration flow
signed out of my purchases only account and into TF with my primary account.
Tried to re-add my TestFlight internal builds via a new invite email (sent to my work email as this is the "internal" ASC one)
On trying to accept the invite I get the message: "The apple account you are currently signed with, [1] does not matching the one which is associated to this invitation. You can sign in to TestFlight with the original Apple Account, [2] or request new invitation from the developer"
where [1] is my primary account I am signed in with and [2] is the old now defunct purchases only account. Note, neither of which is the actual address the invite was sent to.
@nnagesh I have no idea how to resolve this, did you manage to find a solution?