The Magic Circle spell (which takes a minute to cast but lasts an hour) can be used in an inverted way:
When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.
Many planar entity conjuration spells, such as Conjure Woodland Beings, last up to exactly an hour, at which point the creature vanishes:
A summoned creature disappears when it drops to 0 hit points or when the spell ends.
Imagine the following scenario: Bob the Druid casts Conjure Woodland Beings, which makes DM-decided fey creatures (let's say 2 quicklings) appear, and maintains concentration on it. After 30 minutes, Dylan the Wizard decides to cast an inverted Magic Circle around one of the feys, which Bob orders to stand still. After that, Bob drops concentration on Conjure Woodland Beings. What I'm wondering here is what takes priority between "disappears when the spell ends" and "preventing a creature from leaving the cylinder".
Does an inverted Magic Circle prevent a Conjured Creature from vanishing at the end of its spell's duration?
If so, that means that the now-trapped fey is no longer friendly to / controlled by Bob, but remains there until the Magic Circle itself ends (total of 1 hour 31 minutes). If not, that means that the fey disappears as soon as Bob's concentration ends, regardless of the Magic Circle (total of 31 minutes).