I actually had 'Linear - Oldest First' selected, but I changed it to 'Linear - Newest First' and then changed it back again. I'll see if that does anything. Right now there aren't any unread threads in the right column for me to test with.
BTW, to ME 'oldest first' would mean 'always go to post 1' - that being the oldest post in the thread (which is exactly what it's doing).
'Oldest Unread First' might be more clear, if that's the functionality that's supposed to be in play here.
2013 Winnebago Sightseer 36V