This is still happening now with C8's. I only just now started trying to use the slideshow feature with images from a SD card but I see the following issues:
Note that I see this on my two C8's - one is red, one is black (I don't know what else I can say about them...) both are running the recently released control panel. I have different SD cards, but they are mostly class 10, 8GB.
1) checking the checkbox for clock overlay doesn't always work. After I've checked it, sometimes the clock appears, sometimes it doesn't. I always also check the checkbox to shuffle the transition effect.
2) Eventually (through no clear repro steps) I get it to show the clock
3) After some time, one or both of two problems happen: clock stops updating, images stop cycling.
I'll try to pin down more of the repro or results and report back here - i know this isn't super useful now, but it seems like this is still a real thing.