Mar 27 2021
Unless I have misunderstood, Caoimhin has provided a link to a suitable test page that reproduces this fault, as have others - consistently and across platforms in my experience. This should allow someone to narrow down where the fault is. But it is not possible to provide the small test case page that you request, because it does not happen on small pages. It happens towards the end of exceptionally long pages. That is why you will find it for any really common word (such as a or o) in languages that are near the end alphabetically, such as Vietnamese and Welsh.
Nov 11 2020
The example link shown above is not generating the error for me, and it does not look like the Wiktionary entry is nearly long enough to reproduce what I reported in a duplicate, T267708. I guess the original form of the error may no longer be occurring.