Bug with new Title Tool. Old titler (Ctrl+t) works fine. Can't change font on-the-fly, by pressing "Up" and "Down" keys.
When you press the up and down arrow keys to change fonts in the Font Family menu, the Character panel displays the preview of the current font. Fixed for AE CC2018. Thank You! But in the premiere the problem still remained.
Premiere could have the preview font in the text when the new font text is pre-selected, such as in After Effects.
If I read your description correctly, what you are asking is for Live Preview for the Font Menu in PrPro. If so, that is a Feature Request for Essential Graphics in PrPro. I have noted it as such. Thank you for your feedback.