The blendmode extension doesn't support applying blendmodes from within a symbol to elements outside a symbol. This is in the nature of mix-blend-mode:
The
mix-blend-modeCSS property sets how an element's content should blend with the content of the element's parent and the element's background.
As symbols are a grouped entity the elements withing are not on the same HTML group depth and don't share the same parent. Hence, the blend mode won't apply. Nothing I could do about that, as far as I am aware. You can always blend the entire symbol.