The next question would be: What kind of air filters are you using? I’m the type who replaces it every year. When we last checked under the hood after the service, we unfortunately discovered that it hadn’t been replaced. It was dirty and slightly wet.
This time, I kept it pretty simple: I complained right away, insisted on a refund/credit, and then bought a filter myself from a local auto parts store. Despite having the same dimensions according to the manufacturer’s specifications, it didn’t fit perfectly. I solved this by using a 3D-printed frame. Now the filter fits perfectly and seals properly.
I’m using a standard air filter, nothing with activated carbon or anything like that.
Here is a blog post in German that describes this issue and how it can be solved with a bit of “DIY” using scissors and glue (the blog post is not mine, and it can easily be translated using Google Translate

).