Relatively recent (within 5 years) Ultegra 11-speed set up on internally routed carbon frame.
Rear mech. shifts fine...until front mech. is shifted to the big ring.
When there is tension on front cable, rear will not make it to largest cog, and has trouble downshifting.
EDIT: Just removed and reinstalled cables again, this time using two extra pieces of cable housing to attempt to keep the cables organized while routing. Removed the extra housing once the cables were through. Re-assembled....issue is still present.
NEW UPDATE: Still trying to figure out how increasing tension of the FD (when shifting to big ring) decreases cable tension on the RD. So that when the chain is on the big ring up front, the rear will not shift to the largest cog. Leading theory is there is something amiss with the cable routing as proposed by @Nathan Knutson. Here are some pics. Does anyone see anything suspicious? Missing parts?
MORE RECENT UPDATE: Checked chain sizing (in light of a comment below), using the method of bypassing the rear derailleur and wrapping the chain around the largest cog and the large chain ring. The chain appears to be the correct length.
Is it possible that the cables or routing is a red herring, and my issues is somewhere else, like the rear derailleur itself? ...though hard for me to believe because it shifts correctly when in the small chain ring up front.