I'm having the exact same issue, invited players who have previously played on the server are sometimes being told they are not whitelisted when attempting to join. A workaround I've found is closing and reopening the realm, but this is incredibly inconvenient as I have to be at my computer and it boots everyone off who is currently playing. Been experiencing this issue since 1.18 released.
This still occurs in 13w16a, and does look weird, so would be nice to have it fixed!
I'm having the exact same issue, invited players who have previously played on the server are sometimes being told they are not whitelisted when attempting to join. A workaround I've found is closing and reopening the realm, but this is incredibly inconvenient as I have to be at my computer and it boots everyone off who is currently playing. Been experiencing this issue since 1.18 released.