It sounds wrong because "since" is generally combined with a point in time, but "a few months" is a duration, not a date. Also the first paragraph switches tense forms, which makes it stand out even more.
Rewriting it to "since a few months ago" seems to be the easiest way to fix this, though my favorite way to express the the same thing is "as of a few months ago".
It should be noted that the author, like most people you're likely to interact with in this bubble, is not a native speaker of English. What matters is getting the message across - which they did.
You'll end up not being very productive if you spend your time pointing all of these little slips out.
I have routinely noticed this sort of construction since a few years [1]. Does it correspond to standard usage in other languages? If so, which ones?
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[1] See what I did there? Eh? Eh?
In German you might say "seit drei Monaten", translating word-for-word to "since three months", but meaning "for three months now". The author in this case is French, where saying "depuis trois mois" is perfectly fine (as far as I know).
English, where that construction sounds weird and at least needs some helpers around it to exist, is a bit of an odd one out. It is kind of odd that we can say "for three months" to say that something took three months, but we can't say "since three months" to refer to something that has been going on as of three months ago.
Yeah. That’s my guess, too. The Germans I work with almost always phrase it like this.