Had the same thing a few days ago ... the flight, SFO-ORD showed R8 when you looked at it alone .... R0 when paired with an 8 pm onward flight to East Coast and R8 when paired with a 9 pm onward flight to same city on East Coast.
Spent 30 mins on hold, then another 30 with res agent who finally confessed it made no sense, opened the R availability and upgraded me.
I assume it is a glitch or bug ... hope so.