Why it Happens
Atlassian has announced that the new fabric(V2) page editor will not support nesting in Bodied Macros. The Multiexcerpt macros are rich-text body macros.
See Atlassian's announcement about nesting in the fabric(V2) page editor Roadmap: Confluence Cloud Editor Roadmap
Some examples of Atlassian macros that cannot be nested in MultiExcerpt: Expand, layouts, JiraRoadmap, etc. Any rich-text body macro is affected by this behavior. This behavior is repeated in Atlassian's Excerpt Macro which our MultiExcerpt was based on.
See also Introducing live macros to support a new editing experience in Confluence Cloud in the Atlassian Developer blog. Search for "Deprecation of Nested Bodied Macros" on that page.
Give your feedback to Atlassian about their decision to stop supporting nesting Bodied macros by voting on or commenting on CONFCLOUD-68323.