Can you please give more detail about your workflow (coating method, drying method times, developer, ect. all the details step by step) and working conditions (temperature, humidity)?

The step tablet print you show us has no pure white and you can see tonal progression up to the 21th step. But you still don't have a full or almost full black.

Interesting, never encountered such a case:
- Lack of clear whites but full tonal progression until the last step indicates too much exposure to me. (I think we would agree on that.) If it was chemical fogging we wouldn't see tonal progression, right, only fog.
- Lack of full back could be due some kind of bleeding, but then why it's only under the negatives? I'm totally puzzled.

Have you tried other papers to eliminate if that's a paper issue? Also is your FO fresh?