You don’t have a clue what you are talking about. I’ve practiced criminal law for for 30 years and have 100 jury trials under my belt. Neither POI or suspect have a “legal definition” as neither is defined by statute. They are just words or terms which have their general meaning. Neither has any bearing on how police can conduct their investigation. I still read hundreds of affidavits and police reports every year and POI is routinely used. Using the term suspect too early allows a defense attorney to use the “jumped to conclusions” argument at trial which has been quite successful at trial in the past. That’s the reason police began using the POI terminology.
That makes sense to me.