I am at war with the last remaining alliance. In negociation screen (diplomatic status), I can see that I am at war but the pressure of war no longer changes.
The button to impose a truce is grayed out and the tooltip show that "You are not in war with this empire": see the picture below.

The war pressure gauge is stuck in the middle.

In the current game, even after several tens of laps: no evolution of the war pressure.

Exit and reload: the same

• Steps to Reproduce: Don't know...
• Reproducibility: ?
• Version: 1.5.8 S5
• Game saved: At war or not.sav

This is very weird. I've send your report and save to the QA Team.

Weird  is the right word!

Unfortunately, there are a lot of weird things in the game now ...

I really hope you will continue to fix the bugs as you did with 1.5.7 / 1.5.8 releases