Hello Jesse,
1 - i placed the fonts from Yosemite in the folder /Library/Fonts (not the one in /System) (on a Mac, that is enough to activate fonts, and recently activated fonts are
favored over existing ones), then opened Font Book, and verified that the system had disabled the original font files in favor of the ones i installed, then i cleared the font cache (as indicated above) and restarted.
Note: the issue with HelveticaNeue Bold happens on Yosemite and High Sierra; switching to
semi-bold seems to be ok.
Also: this is intermittent: if you see it on a slide, often going to another slide, then coming back makes it disappear (or reappear)
2 - PowerPoint is where my clients saw it; they were not using other Office apps at the time.
3 - These are the font files taken from Yosemite (from /System/Library/Fonts)
HelveticaNeue.dfont lists its version as: 10.0d36e1
HelveticaNeueDeskInterface.ttc does not list any version (nor do i see it in Font Book), but the file's create/modify date is Sep. 10, 2014
5 - The same issue has been observed on Macs using the French-France locale, and the English-US locale.
4 - Here are some screenshots; the same text, garbled, then appearing normal after coming back to the slide several times:


i hope this helps!