I haven't used exosuits all that much before so I'm not very familiar with the mechanics, but I've noticed a couple of oddities:.
Weirdness part one, when a duplicant returns a suit it seems the dock just discards any oxygen in the both suit tank and the dock.
Weirdness part two, while there is a suit now-magically-emptied in the dock, if you try to refill it with oxygen with a low flow rate, oxygen pumped into it shows up as like 40g but then immediately vanishes on the next tick. So the dock will flash its flash "no oxygen" indicator every other second.
After going back and manually forcing some dupes to switch out the suits while watching frame-by-frame, the O2 wasn't deleted. It's just the UI being obtuse, because it says "No oxygen" if the dock's oxygen storage is empty. Even if the suit has 50kg oxygen.
The weirder part is that freshly installing an atmo suit (regardless of oxygen level) makes it viable to wear, but a suit that has been worn for 5 seconds and returned to the dock (via checkpoint) says "NO O2" and is unavailable to use until it has been completely refilled.
To work around that annoying glitch(?) you can manually eject the suit and request another one. Except if you've got more than one atmo suit floating around it can end up loading/refilling the wrong one. Which was what made it look like ~100kg of oxygen had simply disappeared.
Build an exosuit dock, fill it with oxygen at a low flow-rate and then dock a suit, have a duplicant equip and then unequip the suit.