Why it Happens
Atlassian has announced that the new fabric(V2) page editor will not support nested Tables and it will not support rich-text body macros nested in Tables. The Multiexcerpt macros are rich-text body macros.
See Atlassian's announcement of the change to stop supporting nested tables in the fabric(V2) page editor: CONFCLOUD-58656 "Nesting tables are unsupported"
Some examples of Atlassian macros that cannot be nested in Tables: Excerpt, Table, and Layouts. Any rich-text body macro is affected by this behavior. Our (Artemis) Multiexcerpt macros cannot be nested in Tables.
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.
Nesting of rich-text body macros is supported in Layouts so that may be an option instead of using Tables if you need to use the new fabric (v2) page editor.
Atlassian has announced that they will NOT remove the legacy page editor from Confluence so you can use that editor as a workaround. Rich-text body macros can be nested in Tables in the legacy page editor. If your Cloud instances do not have the capability to create "old" pages then contact Atlassian support directly to request that they add that capability to your Cloud instances.
We recommend that you open a support case directly with Atlassian requesting that third party rich-text macros be nestable inside fabric editor Table cells. Please give your feedback to Atlassian about their decision to stop supporting nested rich-text macros by voting on or commenting on CONFCLOUD-58656 and CONFCLOUD-68323.
Add your vote to our MEPOD-55 "Add a plain-text Multiexcerpt macro to the Multiexcerpt app" if a plain-text MultiExcerpt would serve your needs. The styling of a plain-text macro would be primitive but there are not many options considering the decision by Atlassian to not support rich-text body macro nesting.