position coordinates retrieved from any object on a canvas with
pos = canvas.bbox(object) are returned correctly

but when drawn on the canvas (x0,y0) are correct, but
(x1, y1) are not drawn in the proper positions. x1 has been divided by 2 somewhere and y1 as been multiplied by 2 somewhere.
in order for the bounding box to be drawn correctly
x1 and y1 need to be recalculated as follows

x1 = pos[0] + ( ( pos[2] - pos[0] ) * 2 )
y1 = pos[1] + ( ( pos[3] - pos[1] ) / 2 )

I cannot reproduce. Taking a rectangle as an example,
canvas.bbox(canvas.create_rectangle(5, 5, 100, 100, width=1))
returns `(4, 4, 101, 101)` (rather than `(5, 5, 100, 100)`) because of rounding while calculating the outline.

As a result, running
canvas.create_rectangle(canvas.bbox(canvas.create_rectangle(5, 5, 100, 100, width=1)), width=1, outline="yellow")
we see a black outline inside a yellow one (as to be expected).

Could we please have a minimal script to help diagnose the problem?

Thank you for your response. I have been fighting this issue in a large piece of code for quite some time.
So I wrote a small test script as you asked for and the problem will not reproduce. All behaviour of code is as expected.
I must therefore assume there is no bug in tkinter and the problem lies in my code or with my development environment.
Please consider the issue closed

Even if there is something wrong with the returned values, Tkinter is just mostly a thin wrap around Tk. It does not change the returned value, it just converts it to appropriate Python type (a tuple of 4 integers (4, 4, 101, 101) instead of a string "4 4 101 101").