Yes, I think sap has been cut off to that side of the trunk at some point (outlined in yellow) causing the bark to die. Since then, it looks like the tree has grown scar tissue from the side, which will eventually join-up.. However, I fear for the roots on that side as those won't have been fed by the leaves, so likely died and rotted, hence the lean. I think the girdling root is probably supplying sap to much of the tree now, so removing it would probably kill it.
If the above is the case, I think the best you can do as far as keeping it goes, would be to somehow provide it with substantial support. A tricky one, this.
A trowel in the hand is worth a thousand lost under a bush.
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