Airtight but not airtight with round window blocks

Also, in the 'sealed' part of the station with a door, if I open the door, there is no gust of air that comes out, the air vent still shows green (airsealed) and if you stand in the block in front of the air vent the oxygen stays at 100% (and the block on either side, I made a few changes that the 'airtight' part of the inside of the base expanded to the block directly in front and to either side of the front of the vent.

This is not a bug. This is normal behavior in SE1 for many years, the round window blocks (and sloped windows/armor panels) are considered airtight on all sides in a rectangular prism around the blocks. "Airtightness" is decided by individual block faces, and the slopes happen to be airtight on all sides of the grid cubes that they occupy. If your character's head enters this space there is no oxygen to breathe. It's a tradeoff for allowing the rest of your grid to be pressurized. The important thing to understand here is that O2 is not dropping in your grid, you are just occupying a space where there is literally no air (next to a sloped or round panel/window).

For more information, if you are on PC, use a mod like BuildInfo and press Ctrl + to visualize the airtightness of these blocks to further understand.