I2C master and slave use different power sources
The power sources must have the same common for I2C to work. When you changed one of the power sources to 3.3V, you must make sure the ground of the 3.3V supply is still connected to the ground of the 2.8V supply.

Otherwise this is fine in general. In practice you should check what the high logic level threshold is for your 3.3V slave. If it is too close to 2.8V, then you will also need a level shifter to convert the 2.8V levels to 3.3V levels.