Sure! It may be easier to notice in a longer thread: Compare the following two views--searching for D104227 using the built-in search in a web browser initially finds 0 occurrences in the first one (at the same time it works fine in the print preview and finds 1 occurrence in the penultimate comment, at least at the moment of writing):
The issue is related to the unload-on-scroll behavior of Discourse: When you open a page on https://llvm.discourse.group it doesn't load (or show) the entire thread on one page by default but instead progressively loads (and unloads) partial content as you scroll along.
There's no such restriction in the Mailman web UI since it displays the entire thread on one page by default, even for longer threads, e.g., Mailman 3 Proposal: declare "unstable APIs" - Python-Dev - python.org
Loading the complete thread (displaying all messages) allows the built-in search to work without issues.
Best,
Matt