I struggle to fit these issues into a short but descriptive headline sometimes :)

This issue is a little strange. If you didn’t know any better (like me), you’d expect the location of a user’s mailbox to have no impact whatsoever on the function of ‘Recent’ document history inside of Microsoft Excel and Word, but it actually does.

I found this out the hard way of course, when a couple of staff mentioned their recent lists had disappeared and it co-coincided with their Exchange on-prem to Exchange online migration.

After some digging, I came across this Reddit post: 
Users losing Recent Documents lists in Office 2016 due to upgrade to ADFS. It’s the same problem with a slightly different root cause, and goes into a much deeper technical explanation than what I’ll do here.

The short of it is that the Office applications detect what sort of login you’re using – if it’s Active Directory (AD) or Azure Active Directory (AAD). When that state changes, it uses a different registry path for a few things, including those recent documents.

Without knowing for sure but based on my testing, it must be doing some check to see if the associated account’s mailbox is in Exchange Online or not – and if not, it considers it an AD account. It doesn’t matter if you already have the users in Azure AD, Single Sign on and all that other good stuff set up – the single change of changing the mailbox location to online triggered the change for me.

For an AD account, the history paths are saved in the registry here:

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Word\User MRU\AD_1234567890 (the number on the end is some sort of unique GUID).

For an Exch account, it’s in this slightly different path:

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Word\User MRU\ADAL_1234567890 (again, unique GUID at the end).

In case you were wondering, MRU stands for ‘Most Recently Used’. AD is to do with on-prem Active Directory, and ADAL is (according to that reddit post) Azure Active Directory Authentication Library.

Also note the example above is for Word, there’s corresponding paths for other Office applications such as Excel.

There’s two subkeys below this key, one for File MRU and the other Place MRU.

The good news on hitting this scenario is that the values can just be exported, the path changed and re-imported. To do this, via regedit find the registry key that has the values you want (probably the AD one) and right click > export.

Find the file you exported and use notepad to do a find and replace on all the entires for AD_1234567890 and replace to the new value (which you can find from just looking in the registry).

Now, re-import the registry file and you’ll have all the recent document paths restored.

This should only be a one time problem for migrations, and only for people who had a bunch of document paths saved in there and can’t find where they are easily.