I wouldn't call this a "vulnerability", I'd call it "a thing that can potentially turn into a vulnerability, more often it can turn into an obscure bug, and most often it is just a quirk".
In particular, if my corporate security team started just mass-flagging all instances of "str.lower" as "security bugs" I would be having a talk with their manager about their threshold for what constitutes a "security bug". Their job is arguably to be more sensitive to that than most engineers, but not that sensitive. It would be like flagging all instances of string concatenation as a vulnerability... and I say that as the guy who would like to eliminate simple string concatenation from programming languages, already a very extreme position on that operation, because of it being at the root cause of so many vulnerabilities... but simply flagging every use as a "vulnerability" is way too sensitive. A demonstration of the ability to use it to bypass some sort of security barrier is necessary to call any specific instance a "vulnerability".
And string concatenation has caused orders of magnitude more actual, verified vulnerabilities than incorrect case folding has.
What’s a way to flag to an engineering team that they should do a thorough review of their usage of a particular API because it has footguns in it?
This is a rhetorical question because there isn’t a generally accepted way of doing so. Automatically patch everything is a silly way to do vulnerability management but software is cheap to change, so it’s often easier at scale to just force engineering teams to patch even if it doesn’t make sense in context.
I’m not a fan of this approach, but I can understand why it’s so popular.