If it's not showing up like that on any other french channel, TVA may be having some issues broadcast side. Were the captions appearing correctly previously?
There are a bunch of Closed Captioning options on Optik, in both Digital and Standard formats. Without some idea of which of the 5+ options in use, it is difficult to offer comment.
Digital CC option 1 on Optik does not show the artifacts you describe in the few minutes I viewed the channel.
Have you checked if there is the ability to change the CC options on the PikTV device?