Section (or subsection) headings on wikis are usually under the format === Heading 1 : 1 === with only one space between the text and the equal signs.
Step to reproduce:
- Find an article that has a section whose heading contains the following sequence "space, colon, space".
- Edit any section of it using the visual editor and save.
All section headings containing the given pattern, will have "gained" an extra space before the ending equal signs. If you do it again, another extra space appears, and so on. It seems that sometimes the same VisualEditor removes the spaces, probably when editing the top of the page.
Examples seen on various articles edited by different users on the French or English Wikipedias : this edit on frwiki and the next two ones, this other edit and the following four, this one on enwiki, etc.
Reproduced in a sandbox on enwiki : here and several tests between these versions. Spaces only removed in this edit.
This bug does not affect page rendering but can unnecessarily disturb the reader of the difference between two versions. I am not sure but haven't noticed such a behavior before August 2020.