Shimano 105 5800 rear shifter: intermittent downshift jam, clears when held upside down

First post here. I can’t seem to figure out a strange problem with my right-hand Shimano 105 5800 shifter, which is only 15 months and a few thousand miles old.

Initial symptoms on the bike:
Intermittently, when attempting to downshift to an easier gear at the rear (larger sprocket), the large lever would jam just at the point where you would normally expect a click and a shift. No shift happens. This would occur, seemingly at random, in all rear sprockets. The feeling at the lever is the same as hitting the limit stop when you reach the biggest sprocket. Temporary, unreliable remedy: shift up first to a harder gear, then shift down two gears, and hope it doesn’t immediately jam again.

My first thought:
Must be the gear cable fraying. These shifters have a habit of eating the rear derailleur cable inside the shifter mechanism. I’ve had that before, although the symptoms were different: that was a gradual degradation in shift performance, not a total jam. Anyway, I replaced the cable (inner and outer) for good measure, but the problem persists.

Further investigation:
I took the lever off the bike to check the mechanism function without a cable, and the problem is still there. The fault remains exactly as it first presented on the bike:

• occasional jam when attempting shift to larger sprocket
• shift to smaller sprocket always works perfectly, and sometimes temporarily cures the downshift jam

Most oddly, when the lever is held upside down (lever sticking up in the air), the shifting is perfect (at least, once you’ve downshifted to get it unjammed, it stays unjammed).

I have taken the bottom inspection plate off to have a look at the inside of the shifter, but avoided further disassembly (so far, at least…). The only thing I’ve done is clean it out and relube it.

Video:
Youtube video demonstrating

• fully correct operation when held upside down
• then turning it right side up, and successful upshifts
• then the downshift jam occurs
• then turning it upside down again; the downshift jam is maintained
• lastly, an upshift restores fully correct (upside down) operation

Photos:
These are taken in the “jammed” condition (i.e. will not downshift).

Any ideas?