Peculiar behaviour of tea boiling as milk+Water
Original Hypothesis

As mentioned in this post, milk will overflow thanks to the effect of surfactants, which allow for the formation of stable bubbles. Since tea contains both milk and water, it contains surfactants from the milk, but at a lower concentration thanks to the water. This would result in some bubble formation, but not as much as you would find in pure milk, explaining why the tea filled up the container, but did not overflow. Either it was a coincidence that the bubbling stopped right at the top of the container, or it relates to the fact that more surface tension is required to create bubbles without the support of the container walls. In addition, other components in the tea could be contributing to this effect, but I think it is a result of the fact that milk and water are mixed. If you want to experiment with this, I would try a different volume of your water/milk mixture and see if that also makes it to the top of the container without overflowing.

Results of Additional Experiments

The results of your additional experiments seem to indicate that something else is going on. It is interesting that you found a different ratio that filled the container when there was no tea involved. This seems to indicate that the tea is at least somewhat involved in this interaction. Also, the fact that past the ratio of 1:5 (milk:water) the container did not fill up with bubbles seems to indicate that my hypothesis about surface tension was wrong, and that there is nothing special about filling the container. In this case, you probably just got lucky that your original tea did not overflow.

These results are dependent on the fact that you used the same vessel for your new experiments and allowed adequate time for bubbles to form. More information on your experimental procedures will help me narrow down on a possible explanation.