Another user seems to have solved it with alacritty-git. There is still a pending problem on maximized window but the opacity seems ok.
EDIT:
Which version of alacritty do you have?
I have community/alacritty 0.9.0-1 which works fine but uses a different parameter:
background_opacity: 0.6 works fine for me.
chaotic-aur/alacritty-git 1:0.10.0.1891.ge648aae0-1 should, I think, work with opacity (as per your config file above)
The issue might occur because I see, at least in my system, that
/etc/skel/.config/alacritty/alacritty.yml
which is used for the reset, includes the value window.opacity, which is the one used by alacritty-git 1:0.10 and not background_opacity, used by alacritty 0.9.0-1.
So, in my opinion, if you install alacritty-git you should be fine.
Alternatively you could keep the non git version by try starting from scratch (it is all commented) copying /usr/share/doc/alacritty/example/alacritty.yml (which should have background_opacity, to be checked) into your .config/alacritty/alacritty.yml