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I would say that a "best practice" whenever you are going to be requesting a CLI is to have used the card (ideally a good amount) in the cycle(s) leading up to your CLI request. That can definitely be a bit more difficult with a non-major card such as a Care Credit account through Synchrony, as those types of accounts may only be used every once in a long while based on a specific need at a specific time. There are some work arounds for this, though. For Care Credit, one recommendation is to either go to a local Rite-Aid and make a small purchase on occasion or go to your local vet and buy a cheap counter item for < $10 for your pet.
We've heard of CLI requests turning into adverse action, usually CLDs. But usually, the AA is immediate or after a message saying the request has to be reviewed. To approve the request and close the card three weeks later seems weird.
As BBS mentions, CareCredit is somewhat of a different animal. It's certainly plausible that people would request CLIs after non-use in anticipation of a procedure coming up.