Purple Sweet Potato Bread

Hi everyone! Chanced upon Purple sweet potato flour in a local grains store, and thought it'd be good to try baking a loaf!

However, after 2 bakes I've been getting confusing results. Oven spring seems to be lacking, and crumb looks underproofed. Here's a quick breakdown of the 2 bakes:

Bake #1
- 80% Bread Flour (King Arthur)
- 20% Purple sweet potato flour
- 70% Hydration
- 20% Starter
- 2% Salt
- 0.5% Diastatic malt
- 1hr Autolyse + bench kneading (Dough did feel stickier than my usual bakes)
- 7hr BF, 3 coil folds (50% size increase in dough)
- Topped with white sesame seeds
- 1hr proof in banneton at ambient temperature
- 12hr cold ferment overnight
- Baked in dutch oven at 220C (lid on 20mins, lid off 30mins)

Bake #2
- 80% Bread Flour (King Arthur)
- 20% Purple sweet potato flour
- 75% Hydration
- 20% Starter
- 2% Salt
- 1% Diastatic malt
- 1hr Autolyse + bench kneading (Dough did feel stickier than my usual bakes)
- 8hr BF, 3 coil folds (50% size increase in dough)
- Did a preshape
- 1.5hr proof in banneton at ambient temperature
- 12hr cold ferment overnight
- Baked in dutch oven at 220C (lid on 20mins, lid off 30mins)

One thing i also noticed for both bakes is that when i take off the dutch oven lid, the scored section seems to have collapsed? It does rise after 2 mins baking at lid off but I've never had this happening before!

Would appreciate any advice, thank you!