To me, this does not at all look like the picture could be explained by suboptimal filler or lack of processing practice. Rather, it looks as if the boards are "cupping." What does the [Lattentest] say about it?
Hm, you don’t renovate just to then look away!? Then I could have saved myself the effort and the money. If there is a possibility, I would want to improve it and make it optimal under the circumstances.
To me, it does not at all look like the image can be explained by suboptimal putty or lack of processing practice. Rather, it looks as if the panels are "warping". What does the lath test say about that?
I would not assume that at first - no protrusions/unevenness are visible in the test with a long spirit level. Thanks for the hint though!