well, let's see . . .
I've been converting FLAC's using the same source and destination folders, and the same settings inside MC for a long time with no issues.
Updating to MC18 began this problem. Since initial install of MC18 this has occurred each and every time.
Reverting to MC17 the same files converted with no problems.
Moving back to MC18 (even with new and different files) the problem once again is always there.
Not every files claims is reported to fail or tests badly, only some. I've had albums where it was near every file, others it is one or two bad files.
No other factor I can see except the upgrade to MC18.
I have been converting to mp3 outside MC18 since I reported this initially, except for a test when moving to the current build which still showed the problem to exist . . .
But, if it would help I will try MC18 again and send you a sample track of both the FLAC and the bad mp3 that results . . .