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So you have double ended control with one knob controling flow the other temp on the mixer bar?
Sounds possible but may just be the washer slipping.
If you remove the end cap from the flow knob you will see a screw remove that, and the knob will come off revealing the spindle, and its just a normal tap valve.
Turn off the water, use a spanner to remove the valve from the mixer and you can then manipulate it and see if it has threaded or not, but the cost of a valve I would just replace it anyway.
I doubt the spindle is slipping but when you remove it you will see, getting a new knob to sit on the tap spindle probably easier to just replace the mixer tap as they will never match up as the finish will look different being all new and sparkly.
It depends on the type of fixing used, and the type of mixer is the honest answer, some eg ideal standard are just two bolts and they come away, others you have to replace the fixing which can be inside the cavity\behind the tile, in which case I might be inclined to actually use epoxy and glue the knob on the spindle, but as I say the chance of the knob being threaded is unlikely.