Out of curiosity, if you go to the ads catalog and look for SI ads, are there any non-Brave ones there that are set to appear for your OS? If there aren’t, then that would answer you on why you’re not getting them.
https://sampson.codes/brave/ads/my_region/
I just looked for mine and there’s only one ad campaign at this moment that has SI. It is a Brave ad campaign.
If yours is like mine, then there’s your answer. If advertisers aren’t paying to have SI shown, then we won’t see them. Rather than leave us with absolutely no possible ads, Brave will often touch into reserves to pay for ads so we can get something, rather than nothing.