Whenever I enter combat, my health and mana bars appear underneath my character as usual, but the warlock shard indications are gone. I don't know how to get them back.

do you have the option to display combopoints displayed on your target? there's two options for this, one in TidyPlates and one in the blizzard options. Try turning them both off.

do you have the option to display combopoints displayed on your target? there's two options for this, one in TidyPlates and one in the blizzard options. Try turning them both off.

in the tidyplates damage profile menu, in Other Widgets: i have Show Combo Points turned off
and then in the blizzard Names menu, in Unit Nameplates, i have Show Special Resources on Targets also turned off

they were already both off, turning them on has no effect.

I had this problem for months during BFA (about a year ago) and because I was too lazy to set up my UI properly, and a casual player doing old content - I just kept playing without seeing my soul shards. Then I eventually went to displaying my soul shards, though not ideally, through a weak aura. I wasn't even able to detect which addon causes this weird bug, and I wrongly suspected Shadowed Unit Frames :D

Now I'm back to wow, redoing my UI and addons from scratch, and this issue reappeared.

I don't now why and how, but switching the :
Blizzard > Names > Unit nameplates > Show special resources = ON
and
Tidy Plates > Other Widgets > Show Combo Points = OFF
worked for me just fine!

No idea though if it is intended or not, 'cause I find it rather confusing.