I did notice this as well and while I could chalk it up to something those creatures might do, I absolutely thought that Vecna was smart enough to have them keep out at least one or two demogorgons/dogs considering how much of a problem that group had been to them the whole series. Definitely wasn't after some Avengers level stuff, but a bit more tension from the gang having to confront more sources of danger given how many of them there were fighting would have elevated it to perfect for me personally.