Hydraulic tappets will hydraulically "lock" when the plunger is depressed only 0.005 inch, that's how they are designed. That's why the valves opened when you adjusted the lash. With time they will "bleed down" and the valves will close.
One rule of thumb for adjusting hydraulic tappets is to never compress the plunger more than 1/2 of the plunger’s available travel; however it is important to actually measure the travel of the plunger to verify what it is. The plungers of modern hydraulic tappets do not compress as much as the plungers did decades ago. The plunger travel of modern hydraulic tappets is in the range of 0.060 inch to 0.080 inch. This means hydraulic tappet pre-load should be limited to 0.030 inch to 0.040 inch, and sometimes 0.040 inch will be too much.
There are two ramifications to this reduction in plunger travel:
(1) Even though your engine may be equipped with 100% factory valve train, if it’s using new tappets then the OEM length push rods (8.41 inch length) are probably a bit too long.
(2) If your engine has adjustable rocker arms, you need to measure the tappet’s plunger travel and the thread pitch of the rocker arm adjuster, and calculate how far to limit turning the adjuster to avoid compressing the plunger more than half of its travel.
For Example: The push rod cup style adjuster of a T&D rocker arm has 20 threads per inch pitch, which means each complete turn of the adjuster pre-loads a tappet by 0.050 inch. To pre-load a tappet 0.030 inch would require only 3/5 of a turn of the T&D rocker arm adjuster. Knowing this I would pre-load such a tappet and rocker arm combination with 3/10 of a turn, half of the maximum adjustment (i.e. only 0.015 inch).