Hey @one1, thanks for the detailed report and for trying safe mode!
This looks like it could/will be a browser-level autocorrect behaviour — DuckDuckGo’s browser (which is based on WebKit but with its own text handling layer) may be converting the closing " in the BBCode tag to a curly quote before Discourse even sees it. That kind of substitution would break the quote syntax exactly as you’re describing.
Since it doesn’t reproduce on Safari and no other users have reported it, this is likely specific to DuckDuckGo browser’s text input handling, which is outside of what we’re able to fix on our end.
The best path forward for your moderator would be to use a mainstream browser like Safari, Firefox or Chrome. Alternatively, they could check if DuckDuckGo browser has a setting to disable smart/curly quotes.