The only possible way I could see this happening is if the Aliases were "Customer" type (not General / Vendor), and those customers were set to being Temporary (not Active / Inactive).  In that case, the temporary customers could be purged during period / year end, along with all associated data.

Otherwise, it must be a customization / enhancement doing this, or some kind of data corruption in the file (where the temp customer purge is removing more data than it should...).

The only possible way I could see this happening is if the Aliases were "Customer" type (not General / Vendor), and those customers were set to being Temporary (not Active / Inactive).  In that case, the temporary customers could be purged during period / year end, along with all associated data.

Otherwise, it must be a customization / enhancement doing this, or some kind of data corruption in the file (where the temp customer purge is removing more data than it should...).

Thank you. I did check and the types are mostly general and a few for vendor, but no customer. I'm going to do more testing and try in a test company, etc. Appreciate the feedback and will post updates if I have any.

When you say "completely erased", is the IM_AliasItem.M4T file deleted entirely, or just empty (no rows)? 

If it's deleted (in Windows), that's not something Sage 100 programming would do.