When editing the summary inline, the overlay doesn't have a background set, and so content from beneath is visible
This means that the suggestions are hard to read. To be honest, I'm not 100% sure of the utility of having the matching available when you're editing an issue inline (or even in the popup) – I would guess its best use would be when you're creating an issue.
Starting from 1.14.1, similar issues are not shown when editing summary. We have removed this after some customer feedback.