The reason it is acting weird is because that is a scar added object. There used be an option on wall splines that would allow you place a capping object via the edit state, but that has been disabled/removed. The same thing is what happens when you try to variations to other objects.
Examples include, but is not limited to:
wall capping via splines
adding light to post objects
add different building structure details (aka changing a vent to a radar)
This used be all done via DCA variable settings which have been removed (#relicpls), and now require a scar file to get them working correctly.
randomly becomes unselectable with spline tool and the column tends to disappear when editing the shape.
Just to elaborate further about why this is doing what you are describing.
Your object spline is divided up into separate increments (EX 1,2,3,4 and so on) for each separate object that is being used in the spline, this allows you to destroy walls (or edit other variables) via the section, the reason that is is acting weird as you describe is because you are either adding or removing and increment to the spline. This important because the wall cap is tied to the increment inside that spline. So if you were trying to extend it, the cap is going to stay back an increment, and you make it smaller it may outright get rid of it, depending on which end of the increment the cap was. (the cap would always be there if it was placed on the "0" increment, so you might have better luck with one of those)
I hope I managed to answer all of your questions, if you have any more questions feel free to ask me here, via pm, or hit me up on my stream if you see me live, as I do live map making.