Having a single floor of Algae terrariums sitting on tile blocks (10 terrariums all next to eachother on tile blocks). I expanded to have 10 more below those 10. Blueprinted gas permeable tiles over the tiles they were originally sitting on so the other terrariums could contribute to base oxygen. After an hour of not realising why my oxygen was draining in the base I went down to have a look. the room I built below housing the new terrariums with gas permeable tile ceiling above it was at ~3,500g of oxygen pressure, not permeating up. The tile immediately above any of the gas permeable tiles was at ~400g of oxygen pressure. Saving and reloading fixed the tiles and oxygen started permeating properly.
Replace a tile block with a gas permeable tile block.