I've noticed this in a review of the lense:
Image Stabilization (OSS) isn't critical for ultrawide lenses, but its nice to have.
It's not smart; it doesn't know when you're already on a tripod.
If you're on a tripod or resting on a table, turn it OFF otherwise it will blur images made with exposure times around one second!
Can this be the sinner?!... I mainly shoot from tripod and have a shortcut to this on C3 but often I forget about it.. Here is a typical shot made from a steady table with the steady shot on.
I know nothing!
I don't shoot video but I have heard that some modern cameras allow for the vertical format to be shot to cater for those who like to view video on mobile phones.
If you are shooting stills as well as video, is it possible that when you set up for Video you are accidentally setting the wrong format?
I know I sometimes set a wrong setting in various menus.
good luck with finding the cause/solution.