header transparency remove fade effect when scrolling

I am starting a website develloppement and trying to get a sticky transparent header with a semi-opaque color.

I got it to work, but it looks like there is a fade effect when changing from top to sticky on scroll. To be clear, when at top page or in middle of page the header is good, but when scrolling from top to bottom, there is a glitch/fade effect that bring menu transparent for a seconde only.

here is what i done :

– in post set to sticky and transparent
– checked sticky header and set logo and menu color in transparent menu in header setting
– set colors in the general styling setup page for header
– added this code to custom css :

#top .av_header_glassy.av_header_transparency #header_main{
background-color: rgba(24,36,94,0.85);
}

It’s probably missing just a thing, but can’t find where and how to remove that fade effect