One issue is that Frontier decided on a fairly hard limit on one account per mail address, and they tend to lock stuff to email addresses rather quickly. This works ok if you only buy your accounts directly from FD, but when they use other vendors like in this case there will be email clashes.
I had a hard time getting Horizons to work until I realized I have two Frontier accounts, as I accidentally linked my Elite (classic) game to one mail address and Elite Dangerous to the email address I used for Steam.
And the issue here is that when you acquire ED through steam or epic, you are not linking your steam och epic account, you are linking the email address you used when you registered those two different accounts, and that email adress might very well be the same address for many people.