AlwaysPreTouch requires all freshly committed pages to be pre-touched. While currently PS GC does not pre-touch heap pages with -XX:+AlwaysPreTouch.
This issue is related to JDK-8315923, which fixes it when huge pages are used. But the bug still stands if regular page are used. For example, on linux we can reproduce this bug when /sys/kernel/mm/transparent_hugepage/enabled is madvise or never, but cannot reproduce when it is always.

A simple way to reproduce, please make sure large pages are NOT used.
Just run a test with the following parameters
-XX:+AlwaysPreTouch -Xms31g -Xmx31g  -XX:+UseParallelGC
There is no pre-touching during heap initialing stage. After initialization, RSS of the test process is much smaller than memory committed.
On the contrary, using -XX:+UseG1GC and run the test again
-XX:+AlwaysPreTouch -Xms31g -Xmx31g  -XX:+UseG1GC
it takes several seconds to pre-touch heap pages during initialization, and RSS usage after initialization is similar to memory committed. This is the expected behavior of  AlwaysPreTouch

This issue related to JDK-8283935, which uses alignment() instead of os::vm_page_size() as pre-touching step size. The value of alignment() is usually bigger than OS page size, which causes most heap pages are not pre-touched.