This doesn't appear to be zoom related, but it does appear to be a different Chrome bug. It looks that it's not taking into account box-decoration-break when doing the wrapping.
I'm going to confirm this, but this is really just to put together a reduced test case and report it as a Chrome bug.
This is what it looks like on XF 2.1.10 PL2 on Microsoft Edge Chromium 83:
(There are overflows, but my system doesn't show up the scroller by default unless you tried scrolling it.)
Let me assume a workaround for this issue: simply change the content area of the preview content with max-width: 95% or a little bit less.
This can prevent horizontal scrollbar from coming up even if there's a little bit overflow, unless my idea is not robust.
Alternatively, change the inner margin-right or padding-right of the appropriate content level to at least 1.5em as a buffer.
-------------
I tested the following solutions work:
(mentioning @Lukas W. to bring his attention.)
Solution A: .bbCodePreview-content {padding-right: 0.5em;} // if happens again, use 1em in lieu of 0.5 em as a bigger buffer.
Solution B (total solution I'm afraid, though sacrificing the border radius for inline code block):
Don't be so dis-heartened. This is a bug report forum and there are so many threads that I'm sure it's why you haven't received a response. If your solution works I'd like to say nice job.