> ... having this invalid string
That's a cruel test... Well done!
> ... then preview shows, e. g.
> Looks perfect cleaned and canonicalized.
Likewise for me on your example:
> But the dialog shows the non canonicalized path.
> Click to view attachment
Here it can show a /mis/-canonicalized path
> Please look into again.
Let's not be too hasty here. The dialog's live preview has never shown the full path, just the part of the path corresponding to the Format String, validated. I think careful consideration is needed before changing the design intent.