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.
substitute
Replacement.
An appropriate synonym for "supplanted" would be "substitute".
Homophone
Examples of synonyms for the word 'pronoun' are word or substitute.
backup, temporary, another, alternative, substitute
Conflict
option, vote, pick, choosing, alternative, substitute,
Problem
In the UK it is considered a synonym, particularly in the north of the country.
thought
Itβs a synonym. The two words mean the same thin.