Thanks for the quick reply and yep, we tried that and this lead us to point of VTK update to 9.2.2. Since this git history approach did not help us much (i.e., we did not find a specific code change that seems to cause this), we also tried to downgrade our current codebase back to VTK 8.2.0 (there were barely any changes required apart from the VTK itself), and the lines started to work again.
Thus, it really feels like that the code does something that was well-accepted in 8.2.0 (although it might be potentially incorrect code [aka undefined behavior] but 8.2.0 may “deal with it”) but it stops working in 9.2.2. But that is just a guess to be honest.
We tried many different solutions how to tackle the issue, including running the application through RenderDoc and checking the API, but our theories were so far wrong and did not lead to solution.
One observation is that the lines with thickness > 1 are apparently rendered using some VTK solution (i.e., not via driver – please correct me if I am wrong), so the cause might be related to this. We also tried to render lines as tubes (using the RenderLinesAsTubes call) but the behavior was the same.