ᐅ Toilet – Removing and sealing yellow lime stains

Hello everyone,

We have been living in our newly built house since January 1st.
In the house, we have two Villeroy & Boch Omnia Architectura rimless toilet ceramics.
Now we have the following problem: after just a few weeks, there are strong yellow/brown lime deposits on the ceramic, exactly where a minimal amount of water dribbles after flushing.
These stains can be easily removed with acid, but after 3-5 days, they reappear completely yellow/brown.
Unfortunately, it looks very unhygienic. Therefore, we wanted to ask if anyone has a tip on how to solve this issue in the long term? Retrofitting a sealant? Polishing? Filing a complaint?
According to Villeroy & Boch, this is completely normal, and depending on the degree of contamination, you have to soak the deposits with acid every 3-4 days for 24 hours.
I think that can’t be right, can it?

Attached is a picture.

Best regards

Also want to type

Hardness level?

Strange, we have a hardness level of 24 and don't see any such marks. The toilet is cleaned only once a week with a mild cleaner. Could the valve in the cistern be not closing properly?