Last night we saw the pipe is clogged at the 90 deg elbow, but I wasn’t up for getting up there to open it up and clean it out.
The elbow isn’t glued in, it’s some phillips head screws and if you unscrew them the joint will come apart for cleaning.
It’s from plastic that machined off as long stringy bits. The screws penetrate a bit too far through the pipe wall and extend inside. They’re inconsequential for normal flow, but machining certain plastics like UHMW yield these long thin strings that hang on the screw tips. AFAIK the only way to clear that is taking the joint apart. There might be a way to cut the screw down just so it holds but doesn’t extend into the interior. Also, that elbow will flow faster if it’s replaced with 2x 45 deg bends.