Interestingly, Plex seems to be covinced that it actually has valid thumbnail pictures. Otherwise there would be no thumb property in here:
<Chapter id="1" filter="thumb=1" index="1" startTimeOffset="0" endTimeOffset="137679" thumb="/library/media/1349392/chapterImages/1" />
<Chapter id="1" filter="thumb=1" index="2" startTimeOffset="137679" endTimeOffset="352769" thumb="/library/media/1349392/chapterImages/2" />
<Chapter id="1" filter="thumb=1" index="3" startTimeOffset="352769" endTimeOffset="707665" thumb="/library/media/1349392/chapterImages/3" />
Have you tried to remux the file using a current version of MKVtoolnix?
Followed by a complete Plex Dance to flush the invalid thumbnails out?
