Thanks for your answer and for the comments of you all.
This is even stranger. The "playback diagnostics" are ok (All marks green)
I only found an app that play correctly FLAC files on both devices (Chromecast and Huawei mobile P20 Pro -Oreo 8.1-) ==> Foobar2000. (I tested about 6/7 apps)
In any case I'd prefer to use Hi-Fi Cast which is -despite these little problems- an awesome app and my preferred one.
BTW, speaking of "playback diagnostics" I had an error with about half a dozen of FLAC files, probably related with the tag version of the FLAC encoder... In this last case these files are correctly played on the phone -my previous one, a Sony Xperia with Android 5- and refused by Chromecast -just the opposite of initially exposed in this thread-
Hi-Fi cast -under Huawei Oreo 8.1- don't play these FLAC files on both: neither mobile itself -because it is a FLAC file- nor Chromecast, probably due to the tagging issue I just mentioned. I'd need to take more tests.
Thanks in advance for your kindness,
JF