Steps to Reproduce:
Recently on Portuguese Wikipedia, I made minor edits in the articles Timerosal, section Autismo, and Dialeto recifense. In the former, I replaced the phrase "incidências de autismo" (autism occurrences) by "diagnósticos de autismo" (autism diagnoses), and in the latter, I only added a "carece de fontes" (sources needed) predefinition.
I used the Classic Editor (for source code editing). In Wikipedia test page, even on safe mode, (https://test2.wikipedia.org/w/index.php?title=References_test_page&action=edit&safemode=1), the <ref> tags and everything inside them simply vanish even before I start editing. Clicking "Publish changes" will utterly eliminate all references.
Those minor edits actually resulted in erasure of all references of the Timerosal#Autismo section and of the article Dialeto Recifense, and I received two "last warnings" of a Wikipédia moderator attributing vandalism to me.
Links to problematic differences:
Even on safe mode (tested on test2.wikipedia.org), the <ref> tags and everything inside them simply vanish even before I start editing.
Simply the minor edits, without any damage to references of those articles. On edit page, the <ref> tag and the content inside it should appear normally and be editable.
Update: I discovered the source of the problem: the third-party Chrome extension "Use My Words". Disabling it makes the <ref>tags appear again and allows me to normally edit any non-blocked article and save them with no damage. In other words, the bug is not on Wikimedia/Wikipedia, but in a glitched Chrome extension. So this topic can be closed.