Yes, if like most apps, it normally will use WiFi unless unable to make the connection and it will try cellular. That means if your WiFi could not connect for the exact one second that the app tried to make some handshake, it will go to cellular data. There are LOTS of reasons that even good WiFi at home will have momentary outages. Some of them related to the WiFi itself and some due to your internet. Hate to be the bearer of bad news, but nobody at home has 100% perfect internet service. Remember, many ISPs brag about how highly reliable their service is - as much as 99.9%. That’s a minute and a half of down time per day. That’s terrible. By comparison, our data network at work (not an ISP) requires 99.9999% for each route - and we only accept that if there are multiple routes that each have that level of reliability.
If you really don’t want to use cellular data while at home, turn off cellular data.