What is the size of your "hot air bubble" ?
Is it a (small) thermal column of rising air? Then, there, static pressure will remain the same, temperature increases, density is smaller (why it's rising in the first place). Result is altimeter stays constant, and TAS increases if you are to maintain constant IAS.
If your hot bubble is actually the size of an air mass (ie. passing through a warm/cold front), then it's a different answer altogether.