This is a workaround, not a perfect solution.
Golden rule: work on a copy of your document
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Save your document as .fodt (Flat XML ODF Text Document)
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Open the resulting .fodt with a text editor
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A name to be entered in the index is marked up as follows:
<text:alphabetical-index-mark-start text:id="IMark…" />name<text:alphabetical-index-mark-end text:id="IMark…" />
where the … stands for a unique numeric id,
or, in case you defined specifically the key
<text:alphabetical-index-mark text:string-value="key" />name
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Search for the adequate variants, preferably with a regular expression, to delete the XML elements, keeping the “name” ot replacing the parts you need.
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When done, reopen the modified .fodt with Writer and save anew as .odt.
EDIT 2021-07-29
Your edit tells the index marks are not in the master document (where you only insert the Alphabetical Index), but in the individual .odt subdocument files.
Apply the above workaround to every subdocument files.
If you have any index marks in the own text of the master (in addition to the Alphabetical Index insertion), don’t apply the workaround because you’ll lose the master property (a master cannot be saved as a fodt without reverting to ordinary document). Instead, remove the index references manually.
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