Swift ASCII string fails, but works in Objective-C

I'm converting some code from Objective-C to Swift and I have a very simple function that reads text from a file into a string.

@implementation DecodeFile
- (NSString * _Nullable)readFile:(nonnull NSURL *)selectedFile {
    NSString *utf8 = [[NSString alloc] initWithContentsOfURL:selectedFile encoding:NSUTF8StringEncoding error:nil];
    if (utf8) { return  utf8; }
    NSString *ascii = [[NSString alloc] initWithContentsOfURL:selectedFile encoding:NSASCIIStringEncoding error:nil];
    if (ascii) { return  ascii; }
    
    return nil;
}
@end

If utf8 fails, it attempts ascii. This works perfectly fine, but my Swift implementation fails with an ASCII file that succeeds using Obj-C!

print(try? String(contentsOf: selectedFile, encoding: .utf8)) <-- nil
print(try? String(contentsOf: selectedFile, encoding: .ascii)) <-- nil

print(String(data: data, encoding: .utf8)) <-- nil
print(String(data: data, encoding: .ascii)) <-- nil

// Obj-C implementation
print(DecodeFile().read(selectedFile)) <-- WORKS

Why would this fail, and how can I get the same behaviour from Swift as Obj-C?