I’ve read on one Golf forum a while ago (sorry, can’t remember which one) that the indicated 90 degrees isn’t necessarily the true coolant temperature. The true temperature can fluctuate a few degrees either side of 90 under normal driving conditions when the coolant is fully up to ‘normal’ temperature but if it’s within what VW consider to be the normal operating range then the gauge will still show 90.
It’s when the temperature gauge starts to rise some way above or sits some way below 90 that there’s likely to be an issue - e.g. faulty thermostat, faulty sensor, sudden loss of coolant etc. But as @Adam T7 has said, what you’re experiencing is normal.