Ok Hudster,
I think I figured this out. I think we're using two different versions of ACAD here. Your returning a length prop. and a True color prop. which I don't have in A2K. I started A2k6 and drew the pline without closing it and it looked like the one you showed with the one missing corner. Even after filleting and joining it, it still did not work. I listed the "unclosed" and the "closed" pline and this is what I get:
Command: LIST
Select objects: Specify opposite corner: 2 found
Select objects:
LWPOLYLINE Layer: "0"
Space: Model space
Handle = 1cbc
Closed
Constant width 1.00000
area 25.00000
perimeter 20.00000
at point X=798.62500 Y=965.37500 Z= 0.00000
at point X=803.62500 Y=965.37500 Z= 0.00000
at point X=803.62500 Y=960.37500 Z= 0.00000
at point X=798.62500 Y=960.37500 Z= 0.00000
LWPOLYLINE Layer: "0"
Space: Model space
Handle = 1cac
Closed
Constant width 1.00000
area 25.00000
perimeter 20.00000
at point X=808.43750 Y=960.37500 Z= 0.00000
at point X=808.43750 Y=965.37500 Z= 0.00000
at point X=813.43750 Y=965.37500 Z= 0.00000
at point X=813.43750 Y=960.37500 Z= 0.00000
at point X=808.43750 Y=960.37500 Z= 0.00000
It appears that when you draw a pline and don't close it, but return it to the original start point AutoCAD adds an extra 0 length line for some reason.
I did get it to work however; but it's not pretty.
Explode your pline. Find the one that is a 0 length and delete it. Then use the pedit command and choose the multiple option and select your 4 lines. Then choose the join option and then the close option (don't miss the close option or it won't work). Then choose the width option to set your width and you should be done.
Command: e
ERASE 1 found
Command: pedit
Select polyline or [Multiple]: m
Select objects: Specify opposite corner: 4 found
Select objects:
Convert Lines and Arcs to polylines [Yes/No]? <Y> y
Enter an option [Close/Open/Join/Width/Fit/Spline/Decurve/Ltype gen/Undo]: j
Join Type = Extend
Enter fuzz distance or [Jointype] <0.00000>:
3 segments added to polyline
Enter an option [Close/Open/Join/Width/Fit/Spline/Decurve/Ltype gen/Undo]: c
Enter an option [Close/Open/Join/Width/Fit/Spline/Decurve/Ltype gen/Undo]: w
Specify new width for all segments: 1
Enter an option [Close/Open/Join/Width/Fit/Spline/Decurve/Ltype gen/Undo]:
There are better options such as erase and redraw it closed or us the pljoin command from the expresstools if you have it.
I hope this helps everyone,