As mentioned at T234170#5541720, iPad's highlighting issue isn't normally limited to just Wikipedia site but to other sites and apps as well, which is something that Apple Inc can deal with.
However, in this case, I believe that one particular issue is something to deal with as seen in below GIF images.
|desktop site||mobile site|
As seen in the desktop site portion, the highlight part moves while I scroll the source editing window up or down until the scrolling stops. Nonetheless, the highlighted area goes back to the content that I selected.
I tested out the mobile site; no issue there. I tried other apps and sites;
no similar issue there as well. I can conclude that this issue is limited to just desktop site on iPads, including ones supporting the latest version, iPadOS 13.1.2. Unsure about other tablets, including ones using Microsoft Surface and Android, as I don't have such tablet OSes.
*EDIT*: No similar issue at 2017/Newer wikitext editor, especially while it's still at Beta stage.
*EDIT*: Found out that it occurs as well at Apple devices currently using iOS.
*EDIT 3*: See T234482#5549468