I found the same issue with 8.4.0.
I found that if I tried to count commas in a string, it will return one less than the actual number in the string.
I added an underscore to the end of the string and it works fine now.
It caused us much grief as well!!!!!!!!!!!
{STRING(CountOccurencies|B,)} returns 0
{STRING(CountOccurencies|B,_)} returns 1, which is correct.