After more research I figured it out. It's a font problem. I'm using Verdana in Mail and the Apple character is missing. (I'm using Verdana because it shows the capital of the i character distinct from the lower case L character. For instance, when abbreviating Artificial Intelligence to two letters it's confusing.)

I did more research on this. In the Mail app in a new message, typing opt-shift-k still gives me a square instead of the Apple character. However, if I paste in the Apple character I get from typing opt-shift-k in my Notes app it shows as the Apple character in the mail message. Furthermore, after that, if I type opt-shift-k in the Mail message I get the Apple character and can do that as many times as I like. Opening a new Mail message repeats this cycle again starting with the square.... Definitely something I don't understand here. Sonoma 14.3.1, M1 Mac mini. Apple Bluetooth keyboard with keypad and Touch ID key.

FIXED ! At least for me. I just had this same problem come up, with the errors not showing while editing, and then showing up for some number of seconds after attempted build and then disappearing. M1 Mac mini, all the latest software as of today 28 Mar 2023. I discovered it was only the project I was working on. Opening another project worked fine. I started a new project and copied all the text from the files in the problem project over. All good now. I'm too new at this to know how to mark this as a solution to help others. Maybe someone has a moment to school me a little.

Thanks for the extra tips and education. Always nice to know more. Plus, my original "issue" was so easy to fix:-)

I see my question was answered through the escape sequence not being needed in this case. So that's great. I still don't know how to turn off some warning I don't want to be bothered with, but that's another matter and not a real problem, at least for now. Nice to have this resource here for help and education:-)

I know this is old, but it helped me a lot, so thank you. Interesting to note that in Xcode 12.3 the menus and wording to get to the Sandbox setting are different and to me they don't seem to match what's shown in the Xcode help, either. But from what I read here and a little educated guessing / poking around I got there.

I found my answer (enough for now) in this post https://developer.apple.com/forums/thread/96062 It's a sandbox setting that I had a hard time finding because the menu navigation doesn't look like what the Xcode help shows, although I could pick out hints from that and figure it out. So for my use I disabled Sandboxing for my app, good enough for now. I hope this helps someone else.