What if your employer says “don’t access your health records on our machine”? If you put private health information in your Twitter bio, Twitter is not obligated to suddenly treat it as if they were collecting private health information. Otherwise every single user-provided field would be maximally radioactive under GDPR.

If the employer says so and I do so anyway then that’s a employment issue. I still have to follow company rules. But the point is that the company needs to delete the collected data as soon as possible. They are still not allowed to store it.