I have kept the "Check Grammar" option in my browser On, so whenever I write anything wrong as per US English it gets underlined. This is also the case with "multi".
When I use this word in combination with other complete standalone words like multibillion, multimillion or multithreaded, the word "multi" gets underlined (in Red) which means it's not considered correct grammatically.
But it's not considered an error when used in words like multitude, multilateral, etc. Again lateral is a standalone word here but there seems to be no error.
Is "multi" a wrong word in US English? (The said browser software was developed in the USA.)
This link here provides the details about the word "multi" but all of the words used above are considered correct here.