Yeah, prednisone can cause ulcers and stuff.
And at the same time, prednisone can mask ulcer problems. I only noticed I had an ulcer after pooping black, and I suddenly vomited some blood (gives you a scare that one! ). They said the pain and nausea and stuff you normally get was masked by dear old pred.

It's the same with infections: prednisone gives you a higher chance of infection, and at the same time can mask it if you have it.

I think it's such a routine thing to do for docs if they prescribe prednisone, they don't think about explaining the why's and such anymore.