There have been issues with toggling Automatic Solve on and off after importing DWG or DXF files. I have found SPR 475505 that is similar to your situation. After some testing, the workaround in S-034166 seems to work.
- edit the sketch and box select all
- cut all the entities of the sketch
- exit the sketch
- create a new sketch on the same plane
- paste the entities there
You now will have access to the Automatic Solve settings in the new sketch.
I am having the same problem. I found that I can start a new file, open a sketch and then close the sketch. After that I can go back and it will be fixed.
Tools -> Sketch Settings ->Auto-Solve