I have had off and on issues for years. Also LFF, and right lower back is where the soreness is. Starting last summer it's been worse, especially after doing a practice set with someone that seemed a little heavy handed on deep water starts, missed several starts, he said he was going as slow as he could. It's a compounding thing with my starts. When my back starts getting sore, my starts get worse and my back gets more sore. For reference, today i hadnt skied in about 2 weeks and the starts were easy, didnt miss any...no pain....yet. I've tried all kinds of back support braces. I do core and other workouts off and on (maybe more on than off would help more.) I can out plank anyone (many, maybe not anyone.) I finally got to an orthopedic. They just think strained muscles, tho they did see some arthritis in my back....maybe is complicating things. They gave me a anti-inflammatory and a muscle relaxer. I take those when sore and it helps. Like you, looking for more answers. Not more than 2-3 years ago I could do 2 10-11 pass sets in a day and turn around and do it the next day. Now, not so much.
Getting old sucks......