I couldn't find an exact rule that causes this incorrect behavior, but I tried to reduce it to some simple scenarios to reproduce it:
Input: A folder with certain files -> tried to archive it.
If the folder contains more than 7 files the incorrect behavior appears.
Scenario 1: 7 empty files
Result: The created archive contains a single folder entry with the name of the archive (no matter which was the name of the file)
Scenario 2: 7 files, some empty, some with content
Result: The created archive contains a folder entry with the name of the archive and a number of file entries also with the name of the archive. The number of the entries is equal to the number of non empty files.
Scenario 3: 8 empty files
Result: 7zip Manager cannot open archive and stops working.
Scenario 4.1: 8 files: some empty, some with content, last file (alphabetically) with content
Result: same behavior as described for Scenario 2.
Scenario 4.2: 8 files, some empty, some with content, last file empy
Result: archive is corrupt, the following message is received: "Cannot open file 'archivename.7z' as archive" (7Zip Manager does not crash).