That is odd. Custom namespaces shouldn't be included in Special:WikiActivity and Special:Random, I don't think. I noticed Theory pages popping up in WikiActivity too. This is obviously not something we want, since Howling thread names could potentially contain spoilers. Hence why Category:Contains spoilers was — for a period of time, at least — not in any other categories; we don't want anyone coming across those pages names unless they've actively chosen to go to Howling:The Howling.
If this were tomorrow, I'd guess this was an intentional tech update, but there's no such report. How long have you seen this happening? Maybe send in a Special:Contact/bug?