@Technicolor,

I would start with simply replugging the devices. It sounds as though the connection over your network to your HomeKit/Matter Hub has been broken/interrupted - even though the device is still connected to the Wi-Fi and controllable through the Tapo app. You could consider rebooting your Controller, but is not recommended if you have other devices connected.

Do you happen to have an IoT network or have isolated any devices on your network? Your Matter Controller and Devices should be on the same network and they will not function if one is isolated from the other. 

What does your network topology look like? Since it is a network level protocol, misconfigured networks (such as Dual NAT or having two routers) have a much higher chance of interfering with normal operation.

If none of the above applies, you should reset and add the device to the Controller Again. If the device doesn't connect, you may need to remove it from your Apple Home before setting it up again.

  @P125MatterBad,

I was not able to follow up with OP as they didn't reply and it is not one of the common behaviors that I typically see.

Have you been able to add the device to the Tapo App previously? Doing so would allow for any standing firmware updates to proceed - with some of these updates including stability and setup fixes.

When trying to readd the devices, are you performing a soft reset, or a hard reset to get the device ready for setup?

Is the P125M connected only to HomeKit, or do you have it connected via Matter to another platform? (Apple Home takes up more of the total allowed connected controllers than other platforms)