[2021] Ha! A Workbench schematic suddenly appeared

If you leave items in an in-world object that was NOT player placed, the object still calculates it's loot respawn timer. However, so long as items remain in the object, it will not actually respawn even if the timer hits 0.

Instead what happens is it waits until the player empties the object before immediately respawning loot, which is what happened to you.