Nope. We are on v2016 Standard and have loads of entries in IM_AliasItem and never lose them. Any third party enhancements?

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I've also never heard of this.  And can't understand "how" this is happening.  It would seem to have no relationship between the task and the data table.  

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.

Did you ever resolve this issue?  I have a client that has had the IM_AliasItem clear out every month or so for the last year.  We have had the issue in the 2019 and now 2021 version. We cannot find the cause or solution. I was looking through the activity log and thought it might be period end.  I just ran each module period end in a test company after reestablishing the files content.  Period end on no module impacted the file in the test company.