I am seeing this too after a recent upgrade, version 3.3.0.beta3-dev (61610a61fa).
On Android mobile, both Firefox and Chrome.
EDIT: I thought I’d found a workaround, but it doesn’t persist. Might help troubleshoot, though:
Check “Show subcategory list above topics in this category.” box in the parent category’s settings (“Subcategory List Style” doesn’t matter), then go to the All Categories page, and click on the parent category, and then navigate back to the all categories page. The subcategories in that parent category now show up correctly.
But refreshing the page reverts the fix until the next time you go to the parent category.
Is that anything? Sorry if this is nonsense, I’m well out of my depth looking through the code, just trying to do what I can to help track down the issue