Classical Latin's 6 major diphthongs are clear-cut, phonologically speaking. We know ae is pronounced as one phoneme, such as in [ˈsae̯.pɛ], "saepe." However, we often come across words that have 2 vowels next to each other that aren't a diphthong (ex. Italia, puella). As a Romance speaker, I am curious how this would have been pronounced in spoken Latin.
Spanish and Italian distinguish between "strong" and "weak" vowels. /a e o/ are strong and /i u/ are weak. Combining a strong and weak vowel makes a diphthong. For example, Italia is pronounced as /iˈta.lja/, not /iˈta.li.a/. The vowel combination /ia/ is pronounced like a diphthong /ja/, not /i.a/. This phenomenon reduces hiatus in the spoken language. Spanish uses the acute accent to indicate when hiatus does occur, like in /panadeˈɾia/ (panadería).
I have not yet found any sources clarifying this topic in Latin. In fact, most Latin transcriptions imply that hiatus is standard. The word Italia is transcribed as /iˈta.li.a/. and puella as /puˈel.la/. Was this standard pronunciation in Latin, or more of a formal ideal? I could imagine Romans (especially commoners) diphthongizing these vowels in quick speech, and this could carry over into the Romance languages. Any references from ancient scholars would be extremely helpful.