segmentAlongLine() returning line with a gap in the middle

I’m trying to use segmentAlongLine() to return a portion of a polyline based on percentage in my feature class. It works for most of my lines in the feature class, but in some cases, the polyline returned contains a gap in the middle.

For example, here is the original polyline.

When I create a new line using polyline.segmentAlongLine(0, 0.5, True), I get the following line as an output:

Has anyone else run into a similar issue or know what would be causing the gap? It happens for any value of end_measure greater than 0.27 (27%). I’ve also tried switching the start_measure and end_measure values but end up with the same issue.

I’m using Python 2.7 with ArcMap 10.7.1.

EDIT: one detail I failed to mention is the polyline is the result of a Dissolve with "DISSOLVE_LINES" set as my unsplit_lines attribute.