If you use a redstone comparator to measure how much is left inside a tea maker, the signal doesn't update as long as the comparator doesn't get a block update.
It does work for two cases:

• tea maker gets empty
• tea maker is refilled

But not in-between.

in beta4, the update process of tea makers and pan has been little improvement.
Perhaps, there will be a slight delay to Redstone circuit.

Unfortunately it still doesn't work. It still requires a block update next to the comparator.

It will be fixed in the next update.

Comparator has requested the dedicated process to be updated. It was not sufficient in using the World update method.

Fixed in beta 5.