When did people start using issue as though it was a synonym of and direct universal substitute for problem and why is it not considered slang when done so?

My explanation of the "issue" vs "problem" (vs "matter" - I will expand) synonymical use is the following.

Let's start with definitions of Words:

1. issue - thefreedictionary.com/issue - def. point 4 and 5 seem to be of most relevance

2. problem - .thefreedictionary.com/problem - def. point 2 and 3 seem to be of most relevance

3. matter - .thefreedictionary.com/matter - def. point 6 and 7 seem to be of most relevance

"Issue" is interpreted as "problem" from the above definition. We also use the colloquial phrase "to have issue with sth/sb", meaning that somebody / something is a problem.

Also from the above definitions you can see that the Words have broad spectrum of meaning, and the pointed out parts of definitions seem to overlap, therefore giving a synonymical meaning to the three Words listed.

I would therefore not consider using "issue" in the mentioned context as slang/colloquialism, as this is part of actual definition/meaning scope of the Word.

Unfortunately, I do not know when the Word "issue" acquired the definition of problem as listed in the definition points. That would potentially require some heavy linguistic work on lexicology / lexical taxonomy and evolution.

Hope this answer your question.