The interface for a lot of programs with regards to keywords is to show them as a comma separated string. But the important thing to remember is that they are not stored as such. They are stored as individual, separate items, as in @Romeo Ninov XMP example.
To do this in exiftool, you command would be
exiftool -Subject=Tulips -Subject=Flower FILE
Note that this will overwrite any previously existing keywords.
If you wanted to add new keywords without overwriting previous ones, you would add a + sign before the equal
exiftool -Subject+=Tulips -Subject+=Flower FILE
If you write it as your comma separated list, like this:
exiftool -Subject="Tulips,Flower" FILE
then you are writing a single keyword with the value of "Tulips,Flower".
If you find it easier to write it as a comma separated list, then you can add the -sep option. But you must be careful of stray spaces. For example, using
-sep "," if you try to write "Tulips,Flower, Yellow" (note space after the comma before Yellow) then you are writing a keyword of (space)Yellow, not Yellow.
Read exiftool FAQ #17 carefully for details about list type tags.