Spiders need a 2x2 space to spawn in, but they spawn on 1 block. So they can spawn on 1 block if there is a 2x2 space above it.
That sounds like an issue with your router settings. Make sure your NAT is set to open and/or universal plug and play (upnp) is on.
Spiders need a 2x2 space to spawn in, but they spawn on 1 block. So they can spawn on 1 block if there is a 2x2 space above it.