Based on this support article ID: 338270 there was a known problem with the Power Unified Quest removing All splintered sparks of shadowflame.
My alt mage was affected by this bug and behind one splinter. Since turning in the weekly quest, I have done 2 full raid clears, and 10+ mythic dungeons with no catch up splinter.
It probably looks at the whole Loot history of how many splinters were looted up to each week. I believe because of that “loot history” we will never get the splinter by the automatic catch up system. I have since put in 2 tickets with GMs stating that the catch up mechanic should resolve the issue. This is clearly not the case with the amount of content I have since done with the attempt to get the catch up spark.
Can we be compensated for this problem?