The issue is with VMP signing.
VMP provides a method to verify the authenticity of the browser framework. For browser deployments, this will provide an additional signal to determine if a browser-based implementation is sanctioned by Widevine.
Source: https://www.widevine.com/news
Basically VMP will check browser binary files and verify its checksums.

In order to obtain such signatures for BAS, we need to be approved and get required data from Google. Which is obviously complicated.
https://en.wikipedia.org/wiki/Widevine#Criticism
We will try to find workaround, but there is no 100% guarantee that solution will be found.
Проблема с подписью VMP.
VMP provides a method to verify the authenticity of the browser framework. For browser deployments, this will provide an additional signal to determine if a browser-based implementation is sanctioned by Widevine.
Источник: https://www.widevine.com/news
Для получения лицензии VMP проверяет файлы браузера.

Чтобы подписать соответствующие файлы для BAS, приложение должно быть одобрено Google. Что, очевидно, сложно.
https://en.wikipedia.org/wiki/Widevine#Criticism
Сейчас я работаю над решением, но не могу гарантировать со 100% вероятностью, что оно будет найдено.