'grainy' consistency to my dough after mixing and stretching

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Can anyone tell me why this sometimes happens?

I have spent 25 mins slowly mixing my high hydration dough but it isn't coming away from the sides like it should and it looks grainy.

This happens every now and again and sometimes I feel like it's because I'm over stretching it. can that be possible in a mixer.

Also, I ran out of strong bread flower and used 20% self raising instead. Could this be the issue?

Is it saveable after a rest and a stretch?

Any help gratefully received.