We are using the BRC IIF Creator and have run into the same problem. BRC support has been helpful in trying to resolve this - they are able to upload our IIF file into QB 2021 and see the \n as a line feed. When we import the IIF into our QB's, is just shows the "\n" and does not line feed. Wondering if Chrish2 or anyone else has figured this out? Thanks for any help.
We've found that recent versions of QuickBooks ignore the \n characters in memo fields when importing using the standard flow. Starting with QB 2019 Intuit significantly changed (and messed up) IIF import in an effort to make it better. While that failed fantastically, they've done little to fix it since.
Still, there is a way to make it work again like it used to. See this page for steps, and for a long list of issues with the new import code, which (thank goodness) continue to not exist in the old code, which you can access:
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