Recently had to replace old subfloor that was 1/2" with 3/4" due to spongy spots and heavier hardwood going on top. In areas there is up to an 1"-2" gap between subfloor. Home is built on crawlspace, so no basement. Putting down bamboo with Bostik seal n Grip. Some of the smaller gaps will fill easily with Bostik, but on larger gaps will it be an issue having crawlspace air potentially interacting with wood? or is this such a small space that it being covered with floor, and in some cases baseboard, will be fine? Thanks!

I cover small gaps with sheet metal, like flattened tin cans, or galvanized sheet metal, nailed down. It would be best to cover the bare ground with 6 mil plastic sheeting whether you fill the gaps or not. With an engineered floor it's not critical to fill small gaps in the sub-floor.