Eggs do not expire. Once you have 9 eggs, they will remain until one of them hatches through an incubator. Only then can you get a new one.
I’m guessing either:
- you hatched the egg and maybe don’t remember hatching it since you got a new one (I personally don’t always notice when I get a new one through pokestops if I get many objects during at that pokestop).
- There is a glitch and the 10km one got replaced?
However there is probably no fix for this, you’re stuck with the eggs you have until you hatch them.
To answer your question: no, eggs don’t expire.