I only started to have problems with my mismatched leg length[1] when I started riding recumbents. I naturally stand on one leg or the other, so only get lower back pain when forced to use both (eg. when doing the washing up, and there's a cupboard blocking your knee from bending). On an upright I suppose there's enough scope to sit on the saddle slightly wonky and it all just works. But on a recumbent, the seat keeps your back and pelvis straight, so you're either overextending one leg or underextending the other. Plus you can ride for a lot longer without numb fingers or pulverised genitals, so the less severe problems you didn't know you had come into play.
I solved underextended knee issues by a combination of physioterrorism to strengthen the weak stabilising muscles of the lower leg (I do most of my standing on the other one) and ankling more with the other foot. Which worked well until the achilles started to suffer. Last year I sucked it up and forked out for some shortened cranks[2], which stopped the achilles problems dead. I had to make a conscious effort to train myself to pedal hard enough on the shorter crank, but it all seems to work. I've done more miles in 2020 than any previous year, and the only injury has been from racing on an inappropriate seat.
[1] About 10-15mm, from a break of my right femur as a child.
[2] I wanted to use SPD sandals for touring, which precludes shimming the cleat or orthotics.
