Likely that they don't "trust" your email provider, but there's a lot of strange A/B testing going on behind the scenes of that data harvester.
Setting up a dedicated spare gmail or outlook account solves that issue.
Registering a "cell number" helps too, but it's hard finding a service that offers "free texting numbers" that isn't set up in a way that they can see that it's just one of those services. Wish I remembered what finally worked for me.
Lastly, if you're using a modified Discord app or desktop install, be sparing with the non-standard features. That can flag you sometimes too.
For most servers though, I find it easiest to just access them in browser as a not signed in guest.