This is a so-called “linking semivowel”. It’s typically not perceived as being as strong a sound as “original” syllable-initial /w/, so some linguists don’t like to transcribe it (see this blog post by the phonetician John Wells).
The difference could be compared to the more drastic difference between the pronunciation of /p/ in “keep it” vs. “key pit”; the general principle is that consonants that come at the start of a word, or at the start of a stressed syllable at the word level, are “stronger” than other consonants.
“Linking semivowels” occur after “tense” high vowels or diphthongs ending in a high element; linking “w” occurs after vowels/diphthongs ending in a high back component and linking “j” occurs after vowels/diphthongs ending in a high front component.
Sometimes “linking semivowels” are written with superscript letters, although this is not consistent with the official IPA usage of superscript letters.