Yes, the same problem occurs when connecting manually through Settings. After iPhone (iOS 18.3) reboot, I go to settings and connect to the IoT (ESP32) wifi network. The wifi connections gets labeled as "weak security" and "no internet access". Within a minute, the iPhone connects to another wifi network that has internet access automatically. I then connect to the IoT device again and the connection remains intact.

Thanks Eskimo. For now, I have found a workaround: When I use AccessorySetupKit to add the iOT device as an accessory, and afterwards use NEHotspotConfigurationManager to connect to it, the wifi connection doesn't get disconnected after an iPhone reboot.