I tried the example and used your user.cfg.xml. All the test cases worked fine, except for the third one, where it indeed did not list &FLASHmutexHandle, however, that symbol was not defined in the source file. Maybe there was an additional header file you intended to include?
I did stumble upon a couple minor bugs, in ANSI-C mode, we shouldn't be offering "nullptr" or "new CLASS" as completion options. I will add fixes for that to the 25.0.1 hot fix.