Fix error flash is hidden when updating issue repeatedly

What does this MR do and why?

closes #356942 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

1. mock updateIssuable in app/assets/javascripts/issues/show/services/index.js and give a 422 http status code and a json response like {"errors":["Description sensitive or illegal characters involved"]}
2. edit an existing issue, click save changes button.
3. error flash will be shown, click save changes button again.
4. error flash will still be shown.

MR acceptance checklist

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