I have found in the past that air was able to travel up the thread on the nipple.
Could be that.
Not with an eezibleed - the upstream side of the nipple is pressurised.
To do it properly with an eezibleed you need to crack the nipples open before connecting the bottle to the tyre and then tighten them before disconnecting the bottle.
You can easily do both back, or both fronts, at the same time (you can actually do all 4 together but tend to waste a bit more fluid).