I can't figure out this flour

Hi All,

I was wondering if anyone can help me: I'm at a bit of a loss. This may sound stupid, but I'm having some trouble with a new bread flour I'm using, and it either over ferments really quickly, or under proofs during bulk, despite the dough looking ready. I'm using Marraige's Uppermost Bread Flour, and I'm not sure if anyone here is familiar with it. Marriages are a pretty good brand, and I've enjoyed their flour before. 

Anyway, since I started using it, I've had loaves with very little oven spring, small ears, poor crust, and/or massive holes tunnelling all the way through the loaf, despite the dough mostly feeling and looking ok throughout bulk. 

My current recipe is pretty straight forward:

900g Marraige's uppermost white bread flour (12.7% protein)
100g Strong brown flour (Doves Farm Organic, 13% protein)
700-750g water
200g starter (strong white/strong brown blend at 100% hydration)
20g salt

I usually do a 20-30min autolyse, a slap and fold for about 3 minutes, then 6 stretch and folds every 30 mins over 3 hours, and a small bulk ferment of 30-45 mins (depending on the temperature of my kitchen/the dough: this could be more)

On my first bake with this flour, the dough suddenly became very slack after 3 hours stretch and fold at 74F. I took it out to divide and pre-shape immediately, but It was sticky, and wouldn't hold its shape, and left me with terrible loaves after it's final proof. The second time was the same. Both were at 75% hydration.

I then did a much cooler fermentation, which my starter usually enjoys more, but this time the loaf was hugely under proofed, with a massive hole in the centre. This had around 72% hyrdration. 

Last night I made up a batch, with a colder bulk ferment of 68-69F. I did a 20 minute autolyse, a 3 minute slap and fold, then 6 stretch and folds, 30 mins apart, then a bulk ferment on the counter for 1 hour. Divide, pre-shape, and then final proof in the fridge for around 10 hours. 

The dough felt good, it looked good: it was soft, good extensibility, lots of bubbles, and about a 30% rise. 

This morning it had a small rise in the banneton, the poke test seemed positive, filling in slowly, and leaving small indentations. 

However, the loaves are not great. I'm heartbroken. This is the first time, in a long time that I've had a problem like this, and I usually get wonderful loaves from this method, and recipe. It's killing my confidence. My only saving grace is that the loaves taste ok.

Can anyone suggest anything? Has anyone had similar issues? I am so confused. I hope this makes sense! Pictures below of the under-proofed loaf. Happy to clarify any questions.

Many thanks,
Natasha.