I place the cursor at the beginning of a word and then click on Insert Index Entry. When I try to insert the word it disappears to be replaced by a random one nearby and I find that this has been inserted instead of the original selection.
Read these guidelines and edit your question to provide more information, eventually a screenshot.
Personally, since I don’t know how cursor position is managed, I prefer to select word first, so that index is associated with this selection and keeps the association when I edit my text or the word.
Having re-entered 1st Key details everything seems to be OK again.
If it appears to be a misuse on your part, please close the question, choosing the right reason, suc as “no longer relevant”
Seems not to be a misuse exactly. The system gave no problems at all yesterday and only today did it behave strangely unless I re-typed 1st Key details.