I was going to reply to you the other day because I sometimes have the same issue (I even made a thread about it long ago).
For me the solution has seemed to be twofold: 1) ab work … which it looks like you’re already doing, but also 2) thoracic extension and shoulder mobility. I’m getting a lot better at 2, and that’s helped with the arch.
Basically my entire upper spine was locked into position, limiting the bar to be more in front than it should be, so something had to give. Since the upper back couldn’t straighten to compensate, the low back overextends.
It may be what’s going on for you too.
I haven’t fixed this entirely, and I’ve been doing a lot of things to improve that extension, so I can’t point to a single thing that helped, but I can share what I’ve tried.