Arctic Silver 5 will dry out after @ 200 heat cycles ±, so yes that's perfectly normal. With AS5, the pasty component doesn't actually do much, it's really just a medium to hold the silicate and other components that Do do the thermal transfer work. So when it dries out, nothing really changes, it'll last for years, dry as a bone.
But, if you picture the caulking around a bathtub, that seal is very important. After it does dry, if that seal, same as the caulk, breaks, it'll never stick back again. Time to repaste.
Anything from a minor bump on the case to a cpu fan vibration can make that seal fail, it's not glue, and quite fragile.
AS5 isn't the only paste this happens to, there are some few others, but with AS5 popularity over the last 30 odd years, it's become quite commonplace.