currently, a specification change for search bar in issue view is rolling out.
the new search bar's JQL query has been changed from 'text ~' to 'textFields ~'.
and it blocks to search 'comments' from default search bar.
you can search it adding 'text' field from adding 'more+', however, it slows down the work efficiency, since we need to add text field every time.

Suggestion:

• Change the default search in Jira Cloud to use text ~ 'search text' instead of textfields ~ 'search text'
  AND/OR
• Add a Personal Setting for users to choose the default text search behavior.
• Add option(s) to disable/enable searching in comments / summary / description fields to the search drop-down