Looking at https://byjus.com/english/portmanteau/, because it was easy to find:
guesstimate, mocktail, popsicle, breathalyser, athleisure, bromance, frenemy, tragicomic, docudrama, webinar, Medicare, listicle, fanzine, scromelet, chillax, frappuccino, permafrost, staycation, workaholic...
guesstimate, bromance, and tragicomic include all of each of their source words.
And just by looking at your list, you can see why the norm is to include all of one word - those are difficult to interpret for English speakers. Brunch, spork, and sitcom are well understood, though sitcom is more likely to be seen as an abbreviation than a combination. Probably cronut will be understood too, if someone's looking at one. Motel is obviously related to hotel, plus an inexplicable m. Smog is guessable. Cyborg and Velcro are completely opaque; those are just "words".
count percent
both 5 6.4
first word only 29 24.4 (you said "usually," I said "rarely")
neither 39 50.0
second word only 25 19.2
By the way, I called the code "portmantotal.py" :-)
Cyborg was not opaque to me, because I grew up reading science fiction so was very familiar with the concept of the cybernetic organism. My parents explained the origin of "velcro" to me when I was a kid and thought it was cool that it had two sides.
EDIT: While playing with the data in python, I realized that chatgpt included 8 duplicates in the above list originally. I will fix that and recalculate the numbers.
Also, I was thinking about compound words, like seatbelt, schoolbus, butterfly and marshmallow, where all of both words is included -- I don't consider those portmanteaus. And yet, the "both" category in my list is interesting: "covidiot" works because the words overlap, and "blogosphere" adds an extra letter. "Manspread," in my opinion, is also a portmanteau but it's hard to explain why I wouldn't just call it a "compound word" . Probably either because it's derived from "mansplain" or because it adds a noun to a verb whereas compound words are usually noun + noun.
It doesn't. You might note that e.g. wiktionary categorizes motel as a "blend", but sitcom as a "shortening".
"Blend" and "shortening" are necessary-but-not-sufficient synonyms for portmanteau. Your error here is that you imply "because it's a blend, it's not a portmanteau." All portmanteaus are blends. All portmanteaus are shortenings. Some blends are portmanteaus. Some shortenings are portmanteaus.