It might be a bug, if...wi-fi in that example was rock solid, however, if it wasn't, might be that app is allowed to jump between wi-fi and cellular to maintain connectivity. Is wi-fi assist still on?
"When cellular data is on, apps and services use your cellular connection when Wi-Fi isn't available."
...if needed, that tech note also explains how to know which apps are burning cell data, how to turn on/off, etc.
- If activated, Wi-Fi Assist automatically switches to use cellular data when Wi-Fi connectivity is poor. Wi-Fi Assist is on by default*. This might result in additional cellular data use, which could incur additional charges depending on your data plan. You can turn off Wi-Fi Assist in Settings. For more information, see About Wi-Fi Assist.
*Turn Wi-Fi Assist on or off
Wi-Fi Assist is on by default. If you don't want your iOS device to stay connected to the Internet when you have a poor Wi-Fi connection, you can disable Wi-Fi Assist. Go to Settings > Cellular or Settings > Mobile Data. Then scroll down and tap the slider for Wi-Fi Assist.