Cause of change of tea volume during cooling

I would say this should almost definitely be caused by the evaporation, not thermal expansion/density changes.

The cup itself should decrease it's internal volume as it cools (raising the tea level), but you would need to consider at what times it has what temperatures. It may be possible that the lagged thermal expansion of the cup when hot water is added actually does increase the volume of the cup as the tea is heating the water. To test if this is significant (which I doubt it is, given that the temperature changes aren't extreme), you could always heat up the cup first, removing the thermal lag. This would allow you to eliminate this as the cause of the height dropping.

The thermal properties of water can be analyzed and we can compare how much volume change that we see to the expected volume change of the temperature difference:

At $100°C$, the specific volume is $~1.0434 frac {cm^3}{g}$, while at $55°C$, the specific volume is $~ 1.0145 frac{cm_3}{g}$. This means we should expect approximately 2% change in volume if it is coming only from the change in the volume of the water.

If we look at the volume of the full cup, $12 cm$ tall with $3.5 cm$ radius, we get a volume of $461.81 cm^3$. Assuming that your finger width is 1 cm (fairly conservative estimate, especially considering most cups slope outwards if anything), then we would see a new volume of $423.33 cm^3$. This represents an 8% change in volume.

That's 4x what we calculated from just the change in water volume. This to me highly suggests that a majority of the tea lost is lost to evaporation, not to the volume changes in the tea. This makes a lot of sense, as you say the cup is open, and your office is air-conditioned. This generally will help get rid of excess humidity in the room, aiding with the evaporation.

To be sure this is what is happening, try pre-heating your cup with warm water. This should remove the expansion of the cup as a variable that reduces the water level (in theory it would actually be counter-acting the lowering tea level if you heated the cup to the starting tea temperature). What I expect is that when you do this, you will find you lose even more tea. That is because the pre-warmed cup isn't a heat sink for the tea, so it would provide a bit more opportunity to use the heat for evaporation instead of just heating up the room temperature cup.