Even though the Kasa app is showing data and connection, it still could be the HS300 or your network causing the issue. Many of us have had issues with data dropouts, and the symptoms and the cures seem to vary based on in-house networks. Most people point the KP115s being more problematic, but I have had more issues with HS110s and occasionally with my HS300s. If you want a long read:
For me, the go to fixes have been to repower the smart plug, force a DHCP lease renewal for the smart plug, repower the Sense (at the breaker), and restart the DHCP server (router). None of these has worked 100% of the time. The thing that seems to be working best for me today is regularly pinging each of the problematic smartplugs (for me about 5 or 6 regularly had this issue, the other 7 or 8 didn’t). Not only do I get better insight into their behavior - the pings also seem to either keep them visible to the Sense, or tickle them back into visibility.