My experience is being posted as a possible help for others as I finally found a solution.
I downloaded 3 separate RAR DGG Archiv files of a Handel opera and opened them up with 7Zip to yield 3 sets of flac files, But when loaded into an HDD only the middle album displayed in ROON,
A check on Windows Explorer revealed nothing different about the files so they were deleted and reinstalled but that was not help as 2 of the 3 albums still refused to be seen. Similarly rebooting Roon or even the PC achieved nix.
So, in attempt to be “clever”, the track numbers were altered on albums 2 & 3 and the one big file of 70 odd tracks was created, But that did not help either,
Next move, suspecting there was some obscure difference about some files the NCH Switch software was used to “convert” all the flac files to flac, And to avoid some sort of off memory trick ingrained into the previously used HDD, the converted file was imported into a different HDD → the album of 3 then finally correctly displayed.
That was a frustrating exercise but the problem was finally solved although I really have no clue how or why it arose in the first place as it had never been experienced before in this large collection of over 21,000 albums.