This has been discussed many times on the forum: if the airplane has cargo doors on the left side, the same side as the passenger stairs, it won't load baggages until passengers are gone, to prevent the loader to clash into the passenger stair.
However, in the last update ( I realized now this is missing from the release notes ), we removed this restriction if the gate has a jetway, because in that case, with no stairs, there's no risk for conflict with loaders.
