The custom lingering potion and tipped arrow effect have the same duration as the custom potion when used, even though the item description does display the expected duration.
The lingering potion should have 1/4 the duration of the base effect, and the tipped arrows should have half the duration of the lingering potion.

This is fixed in 2.1.1+1.21.4 and 2.1.1+1.21.1