As it turns out, it was, in fact, a cause of the size properties in the BillboardGui.
Old Healthbar Size:
{0,100},{1,0}
New Healthbar Size (the change that fixed the problem):
{0,100},{0,24}
Thanks for the help! The TextLabel was simply clipping out of the Y bounds of the BillboardGui and disappearing, as the Y scale was not consistent with the X and progressively got smaller when moving away, making less room for the TextLabel.
This pretty much sums it up too: