This is exactly what happened to me.
Except it was not with TikTok but another website. OP had 3 pre-existing TikTok accounts. They decided to replace the emails for their TikTok accounts with Proton Pass / SL aliases. And they got this warning.
Most people do not know that Proton Pass / SL doesn’t allow more than 1 alias per third party website, and it’s a huge problem. What’s worse is that Proton is kind of hiding this fact.
The evidence is that many of their agents don’t seem to know about it unless they are challenged on it, and this Redditor who is about to find out. Moreover, in their marketing on social media, Proton encourages people to brag about how many aliases they’ve created. Some have bragged about having created 1000+. But the secret that people don’t know is that only one alias per website is allowed.