As the title asks.
My understanding of a deque was that it allocated "blocks". I don't see how allocating more space invalidates iterators, and if anything, one would think that a deque's iterators would have more guarantees than a vector's, not less.
The C++ standard doesn't specify how deque is implemented. It isn't required to allocate new space by allocating a new chunk and chaining it on to the previous ones, all that's required is that insertion at each end be amortized constant time.
So, while it's easy to see how to implement deque such that it gives the guarantee you want[*], that's not the only way to do it.
[*] Iterators have a reference to an element, plus a reference to the block it's in so that they can continue forward/back off the ends of the block when they reach them. Plus I suppose a reference to the deque itself, so that
operator+ can be constant-time as expected for random-access iterators -- following a chain of links from block to block isn't good enough.