I experienced it many times up to the latest build (v13.0, b.444): the font shows up in the dropdown list and changes within Sony Titler & Text, but NOT in the preview window/timeline. The sometimes seen tip "use Legacy text" works fine, but comes with limitations. Especially those who use Vegasaur (may apply to other plugins as well) have good reasons to use Sony T&T because many of the Vegasaur scripts work only with T&T and not with Legacy text.
So what I do is this: I copy my text in any open Word doc (btw "Wordpad" that still comes free with any Win installation works as well!), select it, change the font to the desired one ─ and now copy'n'paste it back into T&T.
Voilà - the magic is done: the preview window shows the new font. When you select the text you can change its size as well as its face. Once you made all necessary corrections you just save it as preset. Now any time you need the missing font you simply load the preset and change the text.
Of course, you don't wanna do that with a lot of text media, like subtitles. But this is where, for me, Vegasaur kicks in: I simply apply the saved preset to any selection of text media or even the entire subtitle track and the «hidden» or «lost» font shows up in every text event.
TRY IT, I hope it works for you, too!
Haven't found this workaround in any post yet, but maybe I'm wrong.