hope to hear you soon, thanks.
Hi. For clarity, I am not an employee of support.
I do not see on your site that you have applied the code that I provided above. And in addition to this code, configure Header offset to 0 in template settings. This should help.
Do not forget to completely clean Joomla and your device cache before checking.
And in general. In Helix Ultimate, Header behavior has always been a problem. And Helix Ultimate 2 is no exception. Therefore, we have to look for workarounds.
In Helix 3, Header worked just fine. I do not understand why the team cannot take this decision from there.