No, it's a design flaw because it breaks the design paradigm and the math backs that up. Nearly every damage boost in the game system (Hex, Hunter's Mark, Sneak Attack, Colossus Slayer, Giant Killer, Horde Breaker, etc., etc., etc.) are either once per turn or once per round. Even things like casting spells. Once per round (plus potentially an extra cantrip).
This is how the game is designed. I cannot help it if you cannot see this. For people who do various types of design for a living, it's obvious. The game designers went out of their way to prevent abuse by limiting frequency. Remember the 3E Haste issue. It opened up a HUGE can of worms. WotC learned from that.
.
.
Bless
Flame Arrows
Any spell that grants advantage long term
Any spell that paralyzes
If hold person- fighter-blender wasn't supposed to be a thing, someone done goofed.
If flame arrow wasn't supposed to be a thing, someone goofed bad.