Whole wheat "bitterness" once cold, causes and solutions?

I have had it s few times now, where I will make a whole grain bread (wheat and spelt have been victims). It is fine on the day of baking, but noticeably off in taste on the next day.

It's as if there's a bitterness or something, hard to describe. As if bread could go off?

I usually soak my bran and then add it into the bowl as I do my initial dough mixing. I have wondered if this effect rolls in as a result of bran soaking, or in particular if bran is oversaturated with water and makes a kind of "tea" and some bitterant creeps out of the fibers and into the dough?

My latest dough was still moist and soft, didn't have the texture of stale bread but did have this odd taste.

The only other thing I can think of, is maybe a difference between organic, stone ground flour and flour that has been milled with modern methods.

My last bag of whole wheat was Lidl's finest, £1.20 for 1.5kg, tasted fine. The last bread was M&S organic stone ground and I had this bitter issue. I've only used doves farm organic stone ground spelt so there could be a correlation between stoneground and organic combo that creates this flavour?

But on the other hand, I seem to recall the m&s Canadian stoneground wheat being fine, although that isn't organic!

Has anyone else experienced this problem?