Casting a Bonus Action and Reaction Spell on Your Turn

I recently ran a session where I had a weird spellcasting interaction. A player character (PC) provoked an opportunity attack and used the Shield spell to block it. Then, they couldn’t cast a bonus action spell because they had already cast a non-cantrip. This struck the entire table as odd, so I started looking for a way to fix it.

The Rule

The bonus action spellcasting restriction in 5th Edition D&D provides that when you cast a spell on your turn using a bonus action, you can’t cast any spell other than a cantrip other than a cantrip with a casting time of 1 action:

The reason this struck me as odd is because the restriction seems intended to impose a limit on the PC’s ability to cast a leveled spell with their action. It does not seem intended to impose a restriction on the PC’s ability to cast a…