I have tested this on my end and was able to successfully enable the Headphone EQ without any issues. To help us figure out why it is behaving differently on your system, could you let us know if you see any specific error messages pop up when you try to turn it on?
It would also be incredibly helpful if you could try to reproduce the issue one more time and note the exact local time when you attempt to enable the EQ.
Once you share that specific timestamp with us here, we can pull a fresh set of diagnostic logs from your machine and pinpoint exactly what is blocking the EQ from turning on.