Hello,
I had this same exact problem and i wanted to find a solution and i think i’ve found a sort of hacky solution
https://forum.cfx.re/t/fivem-nui-glsl-poc-dev-resource
gtasnail/fivem-glsl: A POC to run glsl on FiveM client game screen. (github.com)
It uses glsl instead of backdrop-filter (with this you can still use backdrop-filter on square div’s but it’s one data value set to achieve the same thing)
Yeah it’s not a perfect solution but it’s something 
