ᐅ Waves on the ceiling caused by increased indoor humidity?

Hello! We have noticed the following issue in our new build: During this time of year, when the lights are on more often, there are strange waves or "imprints" of the joists visible on one ceiling. These are mainly visible only when the light is on, hardly noticeable in daylight. The ceiling is constructed with counter battens and covered with a panel ceiling. On top of that, textured wallpaper is applied. What could be causing this? Could it be due to increased indoor humidity? We are already running an electric dehumidifier. The indoor humidity levels are currently around 55-60% overnight, and about 48-55% during the day. Thank you very much for your help!

I live in an older building and have waves in the ceiling. The ceiling is basically fine, but the appearance is not very good. The floor and walls could also be redone. The most important thing for me right now is the floor. So my question is: do you really have to start with the ceiling and work your way down, or can you start with the floor? The floor is simply my top priority, and I can’t afford to do everything all at once.

There are many attractive options for designing ceilings. The following online store offers such ceiling moldings: link removed by moderation; Bauexperte