Outlook 2016 - controlling IMAP OST size with Group Policy not working


Outlook version
Outlook 2010 32 bit
Email Account
Exchange Server
Hi all, I'm trying the Slipstick Forum as this has been quite helpful in the past. Here goes...
Our previous implementation of Office/Outlook 2013 in a 2008R2 RDS farm ran Outlook in online mode, and allowed users to add pop3, Exchange, or IMAP accounts.
Even though we had Cached Exchange Mode disabled for online mode, we were still able to set the sync slider via GPO by enabling the
Cached Exchange Mode Sync Settings item, and setting it to 3 months. This controlled the amount of email synced if an IMAP account was added, so that end users couldn't sync "All".
We have since moved to new 2019 RDS servers with Office/Outlook 2016 installed. Outlook running in online mode, and we have the GPO settings duplicated, and have verified that the registry settings are present in the User hive
(HKCU/Software/Policies/Microsoft/Office/16.0/outlook/cached mode/SyncWindowSetting=3) and also SyncWindowSettingDays=0)

However, upon logging in as a locked down user with the GPO applied and setting up an IMAP account, the sync slider is set for "All" by default. In addition, I am able to set the sync slider to whatever I want, and it sticks after logon/logoff. Almost like the GPO setting is not recognized at all.

As an additional test to try and see if the setting changed, I set the sync slider for 3 months, and then exported my test user's SOFTWARE hive.
I then changed the sync slider to 1 month, and exported my test user's SOFTWARE hive again.
Comparing these two .reg files, I found a setting named IMAP months to sync in the following path:
Software/Microsoft/Office/16.0/Outlook/Profiles/Default Outlook Profile/9375..........6676/00000003/

It appears that the setting may have been moved to the mail profile instead of the user profile - where we were controlling it before.
Has anyone seen this? I'm sure this is quite a niche case, but it appears that we cannot control IMAP OST sync any more, as it could be in a random key in the mail profile.

Thanks for your time if you've read this far! :)