Thanks for the details, your playback issue is a little perplexing to me right now and I can only suggest you try something.
It appears for your PD16 disk you used DolbyDigital for audio which is no longer available in PD18. In PD18 you used LPCM which is usually very compatible with virtually every player.
It should have no real playback consequence but it appears you must have had to use "Smart Fit" in PD16 for the referenced project as your video bitrate dropped to ~5400Mbps while a more typical ~7900Mbps for PD18.
I'd make sure in PD18 both check boxes in pref > Hardware Acceleration are unchecked and I'd also try the DTS audio and see if you by chance have better luck with that. Any chance you changed burn media? It’s always important to use a good media product for burning DVD's.
Jeff