All of the templates being replaced are invisible/no-rendering templates.

Reproduction steps, or description of what user actions were taken would be very helpful.

In T275650#6857863, @Esanders wrote:

Reproduction steps, or description of what user actions were taken would be very helpful.

I'm sure that it would be useful, but it's just normal editing, and I don't know how to trigger it on purpose. In the first diff, volunteer-me replaced a bunch of outdated content, cited to a variety of outdated sources, with current content and more recent sources. I used the citoid service to generate the new citations. I must have used the PMC id to generate the one that landed at the top, since that's what the URL points to. I don't remember having any unusual problems.

I wonder if this is related to T276164. Some other bug causes the selection to be moved to the beginning of the document (not necessarily the special character toolbar one), and then you end up a template at the beginning without realizing it.

@Whatamidoing-WMF: can you please add a comment on this thread if/when you encounter this again.

As @matmarex noted in T275650#6874514, this may be resolved as a result of T276164.