Because linktr.ee is a shortened domain shared by millions of accounts, and Instagram is wary of shortened, high-traffic domains to begin with. It only takes a fraction of those millions posting spam for the platform to throttle the whole address, and your page gets caught in the sweep even when it's spotless.
Pulling the link and re-adding it can clear it for a bit, but you land right back on the same domain, so it tends to return. The version that stops coming back is a link on a domain with its own clean history instead of a shared one. Something that's been trusted since 2005 doesn't get swept up the way a newer shared address does.