[DIMCURVE isn't a native AutoCAD command -- is it in some overlay program(s)?]

 

The issue is the extension lines being parallel to each other in Arc-length Dimensions, but only for Arcs of included angle less than 90 degrees, at which angle and beyond they become radial to the Arc, as angular Dimensions' extension lines are.

 

Before there ever were Arc-length Dimensions, various people [myself included] came up with routines to simulate them, using angular Dimensions but imposing a text override showing the length along the Arc.  Being angular Dimensions, they had their extension lines radial to the Arc.  And I agree with the OP that DIMARC Dimensions should, too -- it's silly to have them behave differently at 89d59'59" included angle than they do at 90d00'00".  The symbol that precedes the length value and the format of the length value itself indicate that it's measuring a length rather than an angle.

 

But I have no solution to offer, other than the "fake-it" text-override routines, which have the disadvantage that if you alter the Arc so that the length changes, the text content of the Dimension will not -- you'd have to re-do the Dimension.  Maybe that could be solved in new-enough versions using a Field tied to the Arc length [I haven't experimented], but I assume that would work only for Arc entities, not Polyline arc segments that DIMARC can do.