Hello @tristup
Iāve cleared Cornerstone Style Cache, but it didnāt change the situation. But, removing āstickyā from the two bars in the header resolves the issue, and looks like youāre right. There is something wrong with the sticky header bar and offset calculation. Now I have to figure out exactly what.
Although, I still donāt understand why this was working prior to 5.1.X version of the PRO theme 
UPDATE
Ok, I have deleted my Header and created two blank bars and as soon as I enable Sticky option on any of them, the problems with offset start. I tried to play with ātrigger offsetā, but didnāt help. Iām out of the ideas 
It looks like values, when set for trigger offset do not work at all. The only way my one-page nav is working correctly is if I completely disable sticky bars.
What I also noticed is that āScroll Top Anchorā is missing, although itās configured. It doesnāt show at all.
UPDATE 2:
After further examination, it looks like the whole thing behaves the following way:
I set up my header sticky bar as āRelativeā and it behaves as āRelativeā with other elements in my header.
When I switch to a Page Builder, the sticky bar in headers starts to behave as it has been set on āAbsoluteā and covers my page content when I click on any one-page menu item. It looks like header behaves like āAbsoluteā always troughout the whole website.
I really donāt know what else to do and I can guarantee you that this was working prior version 5.1.X. I have rolled back to version 5.0.8 where everything was working (and still does). If you would like to check, please use the same credentials Iāve sent in the private note above. You can try and update to 5.1.4 and you will see the problem arises instantly. Thank you