I have replaced the PD seals and the tandem pump because both were no longer working properly.
Now the oil level remains constant.
Here's what else I did:
With this high oil level, it's likely that some oil has been entering the exhaust system, which is putting a strain on the (relatively new) DPF. It was regenerating very frequently and could only reach approximately 25% regeneration.
Then I increased the ash content (which was in the single-digit percentage range) to 30% (which, in my opinion, makes sense if the DPF has been "contaminated" by the oil).
Now I've had some peace and quiet for a few thousand kilometers.