Housemate's Home app not seeing changes in HomeKit, new accessories not visible

Last week I have purchased some new Philips Hue light bulbs. They are added to the Hue bridge, and they are visible in the Home app. I have added them into the correct rooms, and I have changed some scenes in the Home app to include these new bulbs. All working fine.

I have one shared user in my home. He has been invited as a guest and has not been given rights change the Home configuration. The new Hue bulbs are not visible in his Home app. The changed scenes only work for the older bulbs and accessories, but the scenes on his iPhone do not have the new bulbs included. We have tried everything, including: logging out from iCloud on Apple TV and rebooting, switching off Home in iCloud settings on shared user's iPhone, and in the end fully removing him from the home to re-invite him. After he was removed a new invitation has been sent to his iCloud e-mail address. The invitation cannot be accepted (and was listed as "pending" on my iPhone).

Finally, the only thing we could think of is to set up the iPhone as a new device without restoring an iCloud backup. I have sent a new invitation (allowing remote access and changing Home configurations), which he accepted, and since then all devices, accessories and the new Hue light bulbs are visible.

Well, guess what.. Yesterday I have added another new bulb and renamed an accessory. Nothing special. But these changes again are not reflected in my house member's iPhone... 😟

This seems to be a bug, but we cannot figure out why this happens and how to solve it.. I'm hoping someone here can help us out.

We both run iOS 11.3 on an iPhone 8 Plus and iPhone X.