Was the heater control set to DEF and the temperature control set to full high heat during refill?
My haynes manual also troubleshoots the 1- water pump operation by pinching top hose at idle then releasing to feel resulting coolant surge
2-waterpump drivebelt adjustment
3-restriction requiring radiator backflush

A had issues a while back with overheating. Did a flush and new thermostat, viscous fan and radiator cap but still had a temp high reading on the dash.

I thought it might have been the water pump and tried the pinch test but found it very difficult to feel a surge of water. The only way I knew I was going to eliminate the water pump was to see it, so I disconnected top hose from radiator (ONLY when the engine has cooled and no pressure felt in hose) and literally watched the water pumping into a bucket. It's not a great flow until you rev up the engine a bit but just make sure you don't do it for long or you will empty out the expansion reservoir tank.

My issue was different altogether and turned out to be my wiring harness.

Just a thought, but there might be some sort of voltage compensator circuit in the cluster to allow for the different voltages produced by different cars. i.e Some alternators and/or regulators can operate from 12v up to 14.5v. These variances would change the input to the temp (and other) guages.Maybe it has failed.

Have you got a spare cluster available? I've thought about getting one off flebay as there usually going for $60-$70

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1995 E300 Diesel