If we refer to the specification we can read:
A value other than none for the filter property results in the
creation of a containing block for absolute and fixed positioned
descendants unless the element it applies to is a document root
element in the current browsing context. The list of functions are
applied in the order provided.
This means that your position:fixed element will be positioned relatively to the filtered container and no more the viewport. In other words, it's still fixed but inside its new containing block (the filtered container)
Here is a simplified version to illustrate the issue:
.container {
display: inline-block;
width: 200px;
height: 200vh;
border: 1px solid;
}
.container>div {
position: fixed;
width: 100px;
height: 100px;
background: red;
color: #fff;
}
<div class="container">
<div>I am fixed on scroll</div>
</div>
<div class="container" style="filter:grayscale(1);">
<div>I move with the scroll</div>
</div>
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