I'm having the same problem. I had hundreds of manga I finished and hundreds that I wanted to read, and also a good chunk that I was reading, but they all disappeared. I thought they'd come back with time but it's been weeks and nothing's really changed. The only manga that came back to my lists are manga that I tagged, otherwise they're all lost. ......
Nothing you can do here as someone in the same situation. You just have to wait and see what happens
My acc is from 2020 and so far the first few ones i ever added started reappearing, they might get added back in order. Also so far only the 2020 ones have been readded for me so far