Hyd lifters are not designed to be “solid”.
Page 7-
http://toplineauto.com/wp-content/uploads/2019/04/Lifter-Application-Catalog.pdf
Notice the bleed down rates are rated in “seconds”.
From the FAQ section of the Gaterman site:
Can the lifters be noisy on startup after the engine has been sitting for a while?
Yes, they definitely can. When the engine is shut off, some of the lifters may not be on the cam base circles, but may instead be on the lift portion of the lobes. If that happens, the lifter is under constant load from the valve spring holding the valve open. Eventually, the force will bleed the oil out of the lifter, resulting in clearance during the next engine re-start. The engine will clatter momentarily until the lifter refills with oil to take up the clearance. High ratio rocker arms, strong valve springs, and high lift cam lobes all contribute to this situation. In reality, the lifter wouldn't function without its ability to bleed down under load. So, there's no way to completely avoid it.