Since your router is 2.4 only, I assume you're using neighboring signals to gauge 5 GHz. Have you considered that they may be some distance away? It may simply be taking a bit to scan signals that are on the edge of your laptop's broadcast range.
Anyway, to answer your question. No, I haven't had this problem. However, I have occasionally seen it on 5 GHz with my ASUS USB-N53 wireless dongle.