in my home I have a router and a WiFi extender, when the internet connection from the router is weak, almost 0 Mbps (practically no Internet) , Note 10 does not switch to the wifi extender automatically, but I have to do it manually although there is a network available with a good wifi connection (my extender). This is not a bug, but there should be a minimum internet speed and when the minimum is reached the mobile phone should start looking for other available networks and connect to them, and not stay connected to practically 0 Mbps connection.
This would most likely cause battery drain if it were implemented (network speed can vary especially when other poeple are on the network, and so constant dropping speeds will constantly prompt the device to search for another network, resulting in increased battery usage), it is possible but would require alot of optimisation, a big task for such a small feature. Usually to accomplish seamless transfer from 1 broadcasting network to another, you have to set up a mesh network, which is rather expensive. Its what you find around large buildings that have WiFi such as shopping centres and universities. You can try setting your WiFi extender to broadcast the same ssid with the same passphrase, perhaps your device will switch over more readily. Worth a try.