This is in Brave 1.10.93 Chromium: 83.0.4103.106 (Official Build) (64-bit).
All shields are turned off (gray icon).

Oftentimes when I edit the comment, the button is gone after submitting:

After a full page refresh it is back again:

I can't reproduce this bug in Chrome 83.0.4103.116. When I submit the edit there, the icon is hidden for a split second, but then is always back.

In both browsers I see the console error during initial load (example page):

❌ Refined GitHub → more-dropdown → TypeError: Cannot read property 'append' of null
    at appendBefore (refined-github.js:3157)
    at createDropdown (refined-github.js:3171)
    at more_dropdown_init (refined-github.js:3184)
    at async runFeature (refined-github.js:2837)
    at async setupPageLoad (refined-github.js:2845)

however, I doubt it's related, because it does not appear during the submitting.