After some testing rounds (SUoP 0.3.1.1 on MW 1.17/SMW 1.5.7) we found that categories assigned to an article disappear when refreshing the page. Only after the page is saved again, categories are appear again.
Topic on Extension talk:SemanticUpdateOnPurge
SMW 1.5.7?? I thought 1.5.6 was the last in the 1.5 branch?! Also, SMW < 1.6 is not fully supported, the extension seems to work differently from version to version, there were too many internal changes and it doesn't work very well with 1.5.6 for sure, perhaps with 1.5.2 or earlier. Also it seems to work in SMW 1.6.2, can't confirm the category bug there.