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For comparison...
Calc has the same dialogue for cell borders. Here the actual formatting is correctly pre-selected.
To reproduce the bug...
1. Open the side bar (if not already)
2. Click on the blue line
Note that the line width selector open with 0,5pt highlighted instead with the actual value of 6,0pt. (Screen shot)
Impress is affected by the same bug.
Solution:
Actual value should be pre-selected.