@posila

I had the same problem on my Windows 7 PC. After applying the start-up option (in Steam: --force-d3d) the problem is gone.
So, I assume that on my PC OpenGL was used by default, not DirectX. I had no problems with my graphic card since Factorio 12 Steam Version.
My card is the AMD 290X with ...

I am Happy, thank you devs! Now I can play Factorio again. My belt based factory was down to 850 spm from 1100 spm only because of the Compression bug. I did not bother to create workarounds, I just waited and waited and waited and...

A slightly different scenario:

2 groups of 3 yellow belts should (each) compress a blue belt. The left group works as expected, the right one does not. This is against my intuition. If this behavior is not corrected then splitter output priority becomes pretty much useless to compress belts.

A ...

This is my first post in the forum, so I am not sure if this is the right place to report a bug.
In my test base the output priority of the splitter does not work correctly.