interact_plot: Issues with scale_linetype_manual when interval is plotted

I tried to change the line types for interact_plot() using the method described in this post:
https://stackoverflow.com/questions/78166146/change-lines-types-in-interact-plot-function

However, when I do this when the intervals are plotted (or "interval = TRUE"), then using scale_linetype_manual creates an extra entry in the line type legend that is blank and labeled as "NA". This keeps the color and line type legends from merging.

If I set "interval = FALSE" then this issue resolves, so plotting the intervals seems to be the source of the issue.

Reproduce the issue using interact_plot() with interval = TRUE and scale_linetype_manual(), then compare it with interval = FALSE. Trace the interval and legend handling from the interact_plot() entry point; done means no blank NA linetype legend and merged color and linetype legends, while preserving the non-interval behavior.

Written by the indexing model from the issue text.