Why won’t my digestion adjust?

Been 90% plant based for 16 months. My digestion has been a wreck - painful gas and bloating basically daily.

This week doing a low residue diet to prep for a colonoscopy, I’m eating a lot of chicken and turkey, low fiber, so no veggies or legumes.

My digestion is the best it’s been in a long time and bloating is minimal. So I do think it’s the plant based diet that was wreaking havoc on my stomach. But why wouldn’t it have adjusted after over a year? I’d like to continue eating PB after my colonoscopy but how can I improve digestion?