Maybe that is it. Lack of communication from both sides, I don't know either
I understand your frustration, while still having to note that not only WiiM are affected. That doesn't make the situation any better for you as a customer, but it's a strong indicatior that Deezer is the party not communicating with their partners in time.
Anybody integrating Deezer directly into their own products relies on the SDK (Software Development Kit) provided by Deezer. This SDK allows third parties to make use of Deezer's API (Application Programming Interface). Third parties do not (and are not allowed to) hack into the functionality that might be available through Deezer's API. They
must to use a Deezer SDK. If they get access too late or the documentation is incomplete or plain wrong or if there are unexpected dependencies with other software packages or bugs in the SDK itself (to name just a few examples), they have no chance to properly adapt their own software.
Usually, vendors - and not only streaming services - aim to keep their API backwards compatible when adding new features. This way, a third party implementation (like WiiM's) might not be able to make use of the latest advancements, but at least it will continue to work as before. In this case Deezer decided to break this backwards compatibility. There are few good reasons for this, the most likely one are "lazy" developers not wanting to maintain "outdated" code.
Even if we assume that Deezer did have a really good reason to break backwards compatibility,. it's still not a sign of good habits if they don't enable their partners to act ahead of time.
Truth to be told, I have no clue if WiiM could have done something about this change beforehand. Maybe, maybe not. That fact that other vendors are suffering from the same problem makes me think it's rather the latter. The fact that Deezer's own apps have been updated accordingly is trivially true. Platforms that work with native apps (like Ever solo) might not be affected. But this approach comes with other disadvantages. In the end, both ends need to get it right and the necessary information must be provided.