This appears to be a flaw in OpenStudio. If you try to assign a second thermal zone "downstream" after a thermal zone in one of the horizontal lines on the demand side, OpenStudio will do one of two things.

First, the correct thing it should do -- give a warning that this isn't allowed and prevent the user from making that mistake (see below).

Second, the wrong thing it does in your case -- connect the second thermal zone downstream (see below).

If you eventually try to simulate with an HVAC system like this, you will get a severe error that stops the simulation (see below).

I haven't done enough testing to know when OpenStudio works correctly vs. incorrectly, but this may be worth reporting as a bug/issue on their Github repository. It does not appear that a similar issue already exists.

I believe the only way that you can have two zones in series on the demand side of an air loop is if one of them is a plenum zone. Two conditioned zones cannot be in series with each other for OpenStudio/EnergyPlus.