Jun 22, 2016 01:02:52.726 [0x7fff7f984000] WARN - Could not find the placement within media 551:/Volumes/Plex/Movies/The Hours (2002)/The Hours (2002).mkv for chapter number 18
Jun 22, 2016 01:02:52.726 [0x7fff7f984000] DEBUG - Skipped thumbnail for metadata item 610, media item 551 for chapter number 18:6605920
Jun 22, 2016 01:02:52.726 [0x7fff7f984000] DEBUG - Generated new chapter thumbnails for The Hours.Looking at the logs, PMS 0.9.17.2 has the same issue every time. It so happens that chapter 18 is the exact end of the movie, so i'm guessing PMS cannot make a chapter thumb.
PMS XML: https://dl.dropboxusercontent.com/u/13997835/hours/610.xml
Mediainfo export: https://dl.dropboxusercontent.com/u/13997835/hours/MediaInfo.txt