My version is different because I use a later ubuntu release. You cannot "upgrade" PCManFM without upgrading the whole distribution. Of course, there are ways to install a later application release on a previous ubuntu release, but I do not think that 1- it is necessary, 2- I'd like to be playing with that.
If you open /home/<your_user>/.config/pcmanfm/lubuntu/pcmanfm.conf, do you have these under the [volume] section ?
If nautilus mounts and ejects the removable media fine, I suppose the system is doing what is required.
Would you happen to have another user on this computer ? Do you see the same issue ?
Edit : can you eject it by command line ?
You can find the thumb drive UUID with
sudo eject /media/<your_user>/<your_thum_drive_UUID>
It is usually on /dev/sdb1 but that can vary.