Regarding Thai PBS releasing their production on Viu, I found this article (in Thai): https://www.thaipr.net/entertain/3720090
The article details that there is a contract between Thai PBS and Viu to stream Thai PBS productions via Viu, starting with "My Grandpa is a BL Writer". It does not say how many or for how long this contract lasts.
Basically, their express desire is to reach a larger international audience. Viu is a relatively large player in the region "over 66.4 million monthly viewers across Thailand, Singapore, Malaysia, Indonesia, the Philippines, Myanmar, Hong Kong, as well as Australia and New Zealand" according to the article -- so basically South-East Asia. Which makes sense, I guess, as a first step, since the cultural differences might be smaller and the mutual interest might be higher there than globally; even though it makes it harder for potential viewers in other parts of Asia, Europe, Africa and the Americas.