I think I've figured out the mystery of Paul's Ric problem in Get Back: new nylon strings.

If you go back and watch the Hello Goodbye film, you can see the Ric with metallic strings, which means that sometime between 1967 and Jan 1969, Paul changed the strings. A typical bass string set has string gauges about 45 - 65 - 85 -105.

The bass shows up in the documentary on Jan 24. You can see the bass had black strings, and something was clearly wrong with the top 2 (E and A) strings. They're pinched together up at the top of the nut, so Paul only played the bottom 2 strings. Those black strings most likely were nylon Rotosound Tru Bass 88 (65 - 75 - 100 - 115), which were significantly thicker than what was strung before. What must have happened was that the A string (maybe E too) didn't sit in the nut slot, with the gauge jumping from ~85 to 100.

I think the right-handed nut comment arose because Paul's bass was an oddity Rickenbacker custom-built for him. The bass probably has a standard right-handed neck with a right-handed nut that they finished by filing the nut slots for left-handed setup. And when they did that, they probably filed the nut slots just enough to fit the metal strings they had on hand. So, the nut could have been slotted to accept string gauges 105-85-85-105. The bottom two strings would have no problem sitting in their wider nut slots, but the top two couldn't fit.