I fear it's working as intented, but somehow not logical.

When you drink "potion of strength 8min" and get affected by "beacon strength II 15sec" then leaving area of beacon, you have no strength effect at all.
I would expect to fall back to my potion effect 8min (timer of course reduced even of active beaconeffect).

In a gameplay point of view, it can be a dangerous behavior (i.e. at overworld base (with beacon) preparing by drinking potions, go into nether to hostile pigman, but then loose effects which were given by beacon before).