Forced water base board heater

It sounds like the second floor system is split (one zone, but not a single loop of pipe) so that water is circulating in half (or three-quarters), and not in the other half (or quarter).

You either need to look harder (pull the covers off if you have typical fin-tube baseboard) for bleeder valves you are not finding, or cut the system open and add some. You have classic symptoms of air in the system, and there should be bleeders at the high points of the system, since you really can't get all the air out by bleeding down in the basement. If you have any bleeders on the working sections, certainly give them a try to see if there's any trapped air there, even though they are working.

The small-percentages possibility other than that is that the system has actually frozen on that side. Air-lock is far more likely.

IMO, a sloping piece would look awful.

Better to find something that 'explains' the transition. Normally, this wouldn't be an issue if you have a doorway or archyway between the two rooms, so that you would have an architrave that would create a vertical division between the two rooms.

If the two rooms merge, the ideal would obviously been to have discovered the difference beforehand and to cut down the height of the baseboard in the room with the higher floor to end up with a consistent baseboard height. I am assuming that you don't wish to remove any and cut it down (or build up the lower baseboard, if that's possible, for example if it is not molded then you could add additional height without removing what's there).

If it is molded, then I would cut a section of the molding and create vertical 'ends' to both the high and low sections (for example with a mitered joint, like a picture frame), making sure that the vertical pieces join neatly to both ends of the existing horizontal runs, and also to each other, thus creating a division, but one that will hopefully look sensible.