Some residential mortgages may stipulate in the T&Cs that the rate offered is only applicable for a residential property and the bank would have the right to change the rate and remove the tracker if the house is not a residental property but is an investment property.
Also, the detailed T&Cs would be in the mortgage contract that you signed, not the loan offer.
So, maybe your mortgage does not have these rules, or maybe you haven't found the relevant T&C's yet.