The only source I could find is an 19th century book that too based upon mythologies/folklores as the introduction of the book itself says, and that too refer Porus as King of India,
It was a long time ago.
And (in my untrained opinion) a lot of the confusion arises from the fact that there were actually two Porus's.
The Persian one who fought Alexander.
And a close rival, of an adjoining kingdom, an Indian one.
Also note that Chanakya (a contemporary) nor any of the Indian texts mention Porus at all. Most of what we know is from the Greeks. No Hindu references. Rare Jain references.
The Persian texts are understandably all lost now. The Shahnameh has references. Not much else post Islam has survived from the old days.
I maintain.
For a land and people with so little in the way of military success against foreign fighters, forget foreign lands, its important to appropriate a heroic figure, because he was on what is civilizationally close to being your land, and claim him to be yours in the 21st century.
The Persians with 5000 years of warring and empire building history across continents would not fight you on this or bother too greatly.
He was a satrap after all. To put things in the broader perspective across the tapestry of world history. A heroic figure. A nice story. A nice movie too maybe.