Mineral imbalance in the soil they walk/live on can sometimes make a difference.
I never had wall separation issues until I moved here. I had thrush issues with my club hoof horse but never wall separation. "In the old days" my horses were even on grain.
"These days" they haven't seen grain since 2007, get flax, eat a high quality vit/min supplement, on and on and on.
Our ground is very acidic. My Ag person told me that means it's a breeding ground for "things fungal & bacterial" during the high heat/humidity months that we experience. So, depending on soil balance or imbalance dry or wet sometimes doesn't matter.
It's bad enough I fight wall separation on my two metabolic horses but I have to give due diligence to the two healthy hoofed horses to keep their hooves under control; that includes thrush.
If your horse comes in during any part of the day, keeping the shavings clean also helps. I muck manure every day but only change the shavings "as needed" where urine is concerned.
My horses have grid mats on top of limestone crush, so the urine does drain and saves me a day or two ----- except for the club hoof horse who is really prone to thrush. I change his shavings every 2 - 3 days and he's only in his stall at night
I only bring the shavings up because everything little thing you can do to keep the fungus/bacteria out of the separated areas, the faster the hooves will heal.
I hope progress is fast - I don't envy you.