I can't find a definitive source, so this is a partial answer, but the short version is:
Censorship. (well, that, and a combination of complications from translation/language)
Yes, it is very common, and even some Chinese are confused by it.
And it is not restricted to television, it exists in other forms of media, and also, confusingly, doesn't as well, which adds to the confusion.
(Part of that is, whilst there is a lot of censorship, where there is less, it is on the author to self-censor, should they fall foul of someone who takes offence to having a region or place actually named. With strong online algorithms checking for keywords to trigger the authorities, many find it better to be safe rather than sorry)
The way to get around the censorship, is to read between the lines - the article or author might indicate the language spoken, some cultural specifics, landmarks, etc, then you can work out what they mean.
(Note, however, that with The Three-Body Problem, Chinese sources have said the countries are implicitly labelled in the novel, it's just that when they went to production, they censored it. This leads me on to another issue - when the novel was written, the censorship level was quite a bit different. With the current regime, censorship has skyrocketed, so anything pre-2011 is likely to have 'less' censorship (beyond the usual), and post-2012 is likely to be very heavily censored)
Also, part of the issue of whether to censor is the background or source of the story: Generally, if it is factual - and is not state sensitive - you would get the country name; but if the source story is fiction, then any names/places have to also be fictional.
(Aside from state sensitivity, there is also a cultural aspect, US citizens might not think much of a story having Washington D.C. nuked, but to have Beijing named as being nuked is a major insult, so it becomes 'City B' in 'Country Z' that got zapped)
(From Novel Updates Forum - Why do chinese novels make up other names for certain countries?, a forum for novels being translated):
[...], in a lot of modern day novels, any mentions of major people/places are censored. There's multiple ways for an author to censor. They can shorten the city name or literally call it J Province.
For example: 苏城. It's the Chinese name for Sioux City but it's also Suzhou censored. Would you translate it as Suzhou, leave it as Sucheng? It'll be weird to use Su City. Therefore, I have it as S City. Abbrievated censoring is common in these type of novels. [...]
The letters are generally based on the Chinese pronunciations of the country, not the way it's spelled in English. And then there are still tons of exceptions because of the use of nicknames and the like. Generally it's easier to decipher by how the author describes the country rather than going by the letter used.
[...] it's because the writer is trying to avoid getting censored. This applies to China as well, and authors will often avoid mentioning the name of the city or province or even country even when all the readers know what's being referred to. Generally it's a nonissue to name different countries but they're just acting on the safe side. [...]
- 'United States' (Chinese) '美国' (pinyin) 'mei3 guo2' - becomes 'country M'
And from Reddit - What is Country T?
As a chinese,i'd like to say it's normal in chinese literature works nowadays.
True country name is forbidden to exist in novels or tv series.
If the novel is published today,you definitely will not find true country name.
China's film and television industry has some rules and regulations, they can not refer to existing regime's name. Even in some patriotic films and based on true story, for example, Operation Mekong, where Thailand was associated in this incident, they were also censored as "Country T". For historical dramas, if the story is fictional not real history, the characters' names must also be fictional, for example, the drama "The Longest Day in Chang'an", which is a fictional history drama based on Tang dynasty inspired by the video game Assasin's Creed, the characters inside truly existed in history but it is not allowed to use their real name, avoiding confusion with historical facts.