I’ve solved it by putting the following in a css snippet (Settings->Appearance->CSS snippets):
body {
--checklist-done-decoration: none;
}
I tried putting the var in :root first, but that didn’t work.
If you’d like to change the color of checked items, add --checklist-done-color as well. It’s default set to the var --text-muted.
I personally prefer this to the solution proposed in Option to disable strikethrough for checked todo items