Braces are not rendered in MathJax in the preview of a question

I am using Firefox 94.0.1, with no user scripts. I have a few add-ons, which I can list if needed, but, since the behaviour does not always occur, they cannot always be interfering. My renderer is HTML-CSS, with "fast preview" (maybe that is it?).

In the displayed screenshot, taken during my edit of the deleted answer https://mathoverflow.net/a/407851, the definition of $$K$$ uses \{\}, but the preview does not show braces. The braces showed properly when the answer was undeleted.

EDIT: This does not seem to be about whether the question is deleted or undeleted; it seems that the braces do not show during preview, but do show after rendering. That makes it appear to be a duplicate of Braces don't render in preview, but that is reported resolved.

The comments below (1 2) indicate that this behaviour may not be reproducible, and, indeed, it is not now happening for me. However, it just happened again while I was editing another post (just to clean it up before it was closed). Once again, the preview converted \{\} to {}, but they displayed correctly once the post itself was rendered. When I tried editing it again, the problem did not recur.

• It works for me. Are you sure it wasn’t a random one-time glitch? Did you try to type the expression again? Does it keep being like this when you edit the saved version of your post? What browser version are you using? Do you use any user scripts etc.? Nov 6, 2021 at 6:27
• It also shows the brackets for me. Maybe also specify what browser version you are using, and the information under Math Settings > Math Renderer. Mine: Safari 14.1.2 and renderer HTML-CSS Nov 6, 2021 at 15:48
• Thanks for the suggestions, @EmilJeřábek and @‍GeraldEdgar! It seems to be sporadic for me as well; I cannot reproduce it on the original example, but it just happened on another post. I have updated the post with the requested information. Nov 8, 2021 at 0:07