Mildew smell/taste?

Hi all, new baker here looking for some experience and advice. I’ve made about 5 loaves that have worked out perfectly taste wise(shaping needs a bit of work), but I really wanted turn up the sourness of the loaf which I haven’t been able to achieve yet with my previous loaves. My starter is quite new, about 1 month, but has been consistently rising and I’ve been feeding it everyday once or twice.

I learned from reading online that a wholemeal starter, and longer bulk fermentation (or smaller amount of levin to flour in the recipe) can help with this. I also decided to follow this recipe by pantrymama. https://www.pantrymama.com/extra-sour-sourdough-bread-recipe/ The ratio of levin to flour was notably low (20g starter to 500g flour) so I expected a long bulk fermentation.

I tried this recipe twice- the first time I let the bf on the counter go for 6.5hrs, it didn’t noticeably grow but formed some bubbles and I ran out of time basically waiting for it so I cut it short and put it straight into fridge to cold proof for 32 hours. After baking, as soon as I took it out of the oven, it didn’t smell quite right, like mildew, or was a bit musty. As it cooled the strange smell went away but I noticed a slight flavor of it in the bread. The bread itself wasn’t sour though slightly more flavorful than what I’d previously made.

The second time I tried this, I used the aliquot method and waited 11hrs for the bf again on the counter, for the dough to grow about 70%. When I uncovered the dough I noticed that mildew/musty smell again. The structure after bf also felt a bit too spongy but I put it in the fridge anyway and it’s currently still sitting there but I’m considering chucking it out because I’m worried the smell is mound growth.

My questions are, does anyone else have experience with this mildew smell and is it normal after a really long bulk fermentation? Anyone have experience with this recipe and can tell me how long their bulk fermentation lasted (ie was my starter too weak)? Finally, any tips to turn up the sourness of the loaf? Thank you!