Exactly as the titles states; if there's lag while you're soaring via an umbrella, when there's lag, the stamina bar will be drained for the time that everything was frozen, but, you won't have moved forward for the drained stamina.
I mean, using it 500+ feet in the air with full stamina and then lagging to find yourself at zero stamina, falling to your death without anyway to save yourself (for something you didn't fail on), feels bad.
Not sure if this is a server/client communication issue or what.
I can say this issue is probably the reason behind like 3 deaths for me. To the point I just make extra stamina potions in my q slot because genuinely nothing helps against the lag.