Well, it appears on researching the problem, I've solved it myself
The problem stems from situations in which discrepancies appear between the program files (.lsc) and the configuration file (.ini) for a particular vault. The macro definitions for each MyBlock code segment must be replicated and consistent in each of the files.
Problems arise when a user "opens" a program file (.lsc) containing at least one MyBlock code segment from a different vault and then saves it back into the currently open vault.
The fix is to not do those types of copies without copying the accompanying .ini file (MBFile.ini) along with the program file.
I don't know if this addresses every possible situation but it's taken care of the problem I'd been experiencing.
WalterTheProf