I only see that someone has responded to me through the feed on my phone, never through the notifications on the top right. Then, a couple days later, they will pop up for the ones that have been updated in my feed, but not anything newer. If I didn't see it in my feed, it won't later pop up in notifications, even if I notice it through looking at the question again. This has been happening for only about two months now.
Using the Stack Exchange app on a Galaxy S8+
Android is version 8.0.0