Looking at it on my desktop right now, I'm seeing everything I'd expect, for both local and federated communities. Most typically lately, I'm browsing on my phone, but that's just hitting my instance directly via mobile Firefox, not using an app, so I can't imagine that would have meaningfully different results.

Sounds most likely that this is just a perceptual thing where I'm not consciously realizing that communities Y and Z are posting way more frequently than community X, making me feel like I'm "missing" posts from X that are then trivially found when I go to X directly.

I'll keep an eye out for this a bit more consciously for the next little while and see if that's what's actually going on.