Excess oil / holes in cheese when baking pizza
I agree that cheese is very likely the culprit - here is an article on the scientific approach to Cheese on pizza (with an interesting graphic) that can prompt some informed expiramentation:
(Huffpost) The Best Mix Of Cheeses To Put On Your Pizza, According To Science

James and her colleagues analyzed the properties of seven different
types of cheeses — mozzarella (which is the most often used in pizza),
cheddar, Colby, Edam, Emmental, Gruyere, and provolone — and how they
affect these cheeses while baking.
What did the researchers find?
Since cheddar, Colby, and Edam cheeses have “small elasticity,” they
didn’t easily form blisters when baking. As for Gruyere, Emmental, and
provolone, their large amount of free oil prevented moisture from
easily evaporating and so resulted in less browning. Meanwhile,
mozzarella easily blisters.
So, the researchers concluded in their study that mozzarella can be
combined with any of the other six cheeses to create just the right
amount of browning and blistering you prefer on a gourmet pizza — for
instance, try cheddar for less blisters or provolone for less
browning.