PowerShell use regular expression to split a string
In PowerShell when you use a -split function if you have part of the match in brackets () you are asking for that match to be returned as well. I am sure that the same is true with the static method of [regex] as well. Consider the output from the two following commands (which are similar to yours) and you will see
[regex]::split("1,2 3", '(,|\s+)')1,23[regex]::split("1,2 3", ',|\s+')123In the first example you see that the comma and whitespace have been returned as elements. What I am explaining is documented in About_Split
By default, the delimiter is omitted from the results. To preserve all or part of the delimiter, enclose in parentheses the part that you want to preserve.
In this particular case
As pointed out in the comments there are 2 more ideal regex strings that would handle this particular case better
(?:,|\s)+ or [,\s]+
Former using a non capturing group and latter being a character class.