According to the MariaDB documentation:
There are no special operators, and searches consist of one or more comma-separated keywords.
The search clearly does not need to be comma-separated, as replacing commas with spaces gives the same result.
I assume that it breaks the string into separate keywords, but exactly how doesn't appear to be well documented.
With my test data, these two return the same results:
AGAINST('Quality Water Environment' IN NATURAL LANGUAGE MODE)
AGAINST('Quality Water åîøüé!@#$%^&*()_+Environment' IN NATURAL LANGUAGE MODE)
The second search has some characters that I consider to be 'word characters' that seem to have no influence on the result.
So what exactly is accepted by this function, and what is filtered out?
