Steps for reproducing:
Edit multiexcerpt include macro
Select different space in ‘Space with Excerpt’ field (IMPORTANT: the new space should be selected with the keyboard and not with the mouse).
Actual result: options in ‘pages with excerpts' fields are suggested for the old space.
Expected results: options in ‘pages with excerpts' fields are suggested for the new selected space.
Workaround: Use mouse for selecting the space in ‘Space with Excerpt’ field.
Another workaround: reload the "Space with Excerpt" input by retyping/selecting the space.
Fixed in 5.2.20 (Jan01, 2020)