Hi @Chris,
Based on my experience, sometimes issues like this might be caused by 3rd-party add-ins that interferes with Outlook. Given this, you can have a look at both the affected users' Outlook and see if they are using any add-ins in common in Outlook. Try disabling them if they exist and check the result.
If the issue persists, considering that for one of the users, issue started after he got the new computer, you can compare the build number of his Windows OS with other normal users and see if they are the same.
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