Such a scenario is definitely problematic, as the Thread credentials are stored on the smartphone when the Thread network is created (in the encrypted iCloud Keychain on iOS / in Google Play Services on Android). At least if we assume Thread version 1.3. This will change in the future with Thread 1.4 and Matter 1.4.
But let’s assume that you created your Thread network with your smartphone and can therefore add devices to it. The first question that arises is which smartphone/account was used to create the Thread network at your girlfriend’s house. Since the Thread credentials are requested during pairing, I would assume that the network was not created with your smartphone/account. Or the Thread Border Router has been reset in the meantime.
To rule out such problems, I would suggest that you use your smartphone for your smart home in your apartment and, conversely, your girlfriend’s smartphone in her smart home.
Since it works for you, I would recommend that you don’t change anything in your own home. At your girlfriend’s home, you might want to consider performing a factory reset of the hub (border router) and then recreating the Thread network (important), so that the Hub generates fresh credentials and passes them to the smartphone as the new default.
As far as IKEA devices and Android are concerned, there seem to be other problems. But maybe try the following trick:
But as Sugar already wrote, it probably won’t work with the M100, and you’ll have to do something different with the light bulbs, of course. But I would give it a try.
Just because Matter works over Wi-Fi does not automatically mean that Matter also works over Thread. With IPv6 in particular, your devices may have created their own link-local addresses. However, packets with these addresses are discarded on their way to the Thread network, whereas this does not happen in the Wi-Fi network. Be sure to check which addresses are used by the devices in your network. For more information, see the chapter “Link-Local vs. GUA vs. ULA” in the tutorial: Matter & Thread: A Guide Through the Jungle