All my Df dials work the same. Sorry to hear that your ISO dial is having some issues.
I'm just taking a SWAG here, but if it were the weather sealing, I would think that it would be consistently tighter.
The fact that it loosens up, then gets tighter again leads me to believe that there is possibly some foreign substance in there gumming things up or perhaps if they lubricate the post or the dial in some fashion that the lubricant has gotten contaminated.
I'm curious what others think.