At the frame (or near the frame) that a fuel item is consumed, the remaining time becomes a negative value for short time.

This can be reproduced by adding the following code to syncDataToUI function

if ui_state.estimated_consumption < 0 then
player.print(ui_state.estimated_consumption)
end

The negative value is also visible in UI.
Not sure if this is an issue of this mod, or the jetpack, though.

Probably it's on my side. I would guess that I don't properly handle what happens when the fuel item refreshes but just never observed it for myself for.. some reason.