Interesting, I have seen this elsewhere, I believe it was in the SEO description field on a post page or something like that, when saving it would double-encode the ampersand "&" and thus create a mess, because that encoded entity would have an ampersand again, e.g. "&", which would then become "&" and it would get longer and longer with every save of the article/post ending up in "&amp", you get the picture 😜.
So i do believe what is at play here is a similar problem, the post was created containing a colon ":", when saved, that was encoded as ":", then the post was edited (that's where the theory begins) and when saving that edit, the ampersand of the ":" HTML entity was encoded as "&", rendering that first HTML entity not an entity anymore by turning it into ":", get it?
Here's some support for that theory:
https://community.khoros.com/t5/Developer-Discussion/JavaScript-in-custom-content/m-p/93590
