In my opinion you should never be forced to fight underleveled. It should be possible to reach the level of the opponents if you follow the flow of the story without looking under every stone and press xp from everywhere. Who shortcuts or explores too much and faces spiders at level 2, bad luck.
This is partially the fault of Larian for labeling enemies as having a "level" which we'd naturally relate to our character levels. But in D&D you're often supposed to go against enemies that are stronger than any individual member of your party, and have abilities that our characters don't get until higher levels. D&D enemies have different stats (typically more HP but much less flexibility) than PCs, whereas in DOS games typically enemies had similar abilities and were actually equivalent to player levels.
Take a Young Green Dragon, a monster that appears in a certain official D&D module. It is Challenge Rating 8. But realistically, four level 5 PCs should be able to take him down fairly with only moderate difficulty.
Or take the Thug, a CR 1/2 enemy. Should be incredibly weak, no? However, the Thug has 5 hit die and multiattack, things which a level-5 PC would have. Larian would probably label him as a "level 5" enemy, even though a party of level 1 can easily handle a Thug.
