Private WiFi address set to “off” does not stay “off” on iPads and iPhones with iOS18

I have an issue with three iPhones (13, 14, SE) and three iPads (Air4, 2x Pro), five running iOS/iPadOS 18.0. To connect to the home network, the devices need to show their true device MAC, because the router (Fritzbox 5690 Pro) assigns static IP addresses based on the device MAC. Only known devices are allowed on the network, if a device with an unknown MAC connects, the router is set to block it.

When I set the Private WiFi address in these devices to “off”, the devices can join the network. Unfortunately, putting the devices into Airplane Mode, or switching them off and on again, changes this setting in a what seems to me somewhat random fashion to either “fixed” or “rotating”. In consequence, the devices are blocked by the router (which then displays sort of a captive portal on the device, basically stating that a connection time limit has been reached).

At the same time the additional slider “Auto-Login” appears in the devices WiFi settings.

Setting “Auto-Join” to “on”, “Auto-Login” to “off” and the Private Wifi address again to “off” allows the devices to join the network again. However, the next time, the device has been switched off, out of the network for a certain period of time, or been in Airplane Mode, the Private Wifi address is set again to “fixed” or “rotating” and the device cannot connect again, until it is manually set to Private WiFi address to “off” again.

I tried to install a profile onto the devices. This fixes it, but if anyone plays around with the WiFi settings, the profile will no longer be obeyed, but will have to be re-installed.

Does anyone have any ideas, how “Private Wifi address” may be set immutable to “off” for a certain network, in a way that it survives a restart, a longer log-off from the network, or Airplane Mode?

[Edited by Moderator]