Let us know what is was.

If it were me I'd start by trying a different cap as well. They may seem simple, but there are actually a brilliant piece of engineering. Most simple things are.

I'm going to assume you don't know anything about maintenance on this bike for clarity. If you know all this (which is seems you do as you've flushed it) all the better. Other people may not know this though and/or you may have forgotten something.

Than I'd drop the coolant fluid and check if the water pump is turning by turning the motor off by hand with the real wheel. You have to remove the water pump cover off course. While you're there make sure the weep hole is unblocked. BMW fills it up which is the stupidest thing.

Than flush the whole system with a hose (low pressure) and flush it out the RHS of the engine as per the picture. Newer bikes don't have that nipple but a bolt instead. Flush until you get clear fluid and heaps of it. If you're anal you can collect the clear stuff and check it for grit of other crap.

Than re-fill with a nitrate free coolant mix of 50-50. Make sure some passes past the RHS burp bolt and put it back in and tighten when it does. It's at the high end of the system, so it may take some fillings. Rinse off excess coolant off the motor and electrical components when you put the bolt back in.

As you fill the system make sure you squeeze the crap out of the hoses on both sides to push the air our the system. Pour slowly to allow air to escape. Fill it all the way to the neck and put the cap on. Than fill up the burp tank to the max level.

Go through a heat cycle (run till the fan comes on or around the block) and check and fill if required when it's cold.

If there is still an issue, there are other things at play.

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