Flat flex cable soldered to PCB is developing a short circuit over time
A few things that I can see that are probably not helping:


Spacing between the pads is not consistent because of the traces taking some of the space between the pins.  Between pins 8-9 and 11-12 the spacing is quite narrow
The PCB CAD does not show the copper pours up to and between the pins that are present on the physical board, the pad-to-pour distance might have been too small for the board manufacturer to do well
It's not a good idea to create an acute angle when connecting to a pad because this may not etch well, although usually not a source of shorts.
The vias on the ribbon actually make the effective pad width wider, so it may be easy for them to contact the adjacent pin, especially if the space between the pads is narrow.


It's hard to tell what is going on underneath the ribbon- I would assume that the majority of the shorts are actually due to solder spilling between pins under the ribbon.  It's difficult to solder well, so that's where a proper connector is your friend.  You could try desoldering the whole thing, stripping out any copper pours that went near or between the pads, trimming back the fatter pads, and trying to solder the ribbon a bit higher off the board so that the ribbon isn't possibly squishing solder between the pins.  Use lots of flux so that the solder really just wants to be on the pads.