We had a workflow in our existing CRM environment that does validation on the lead entity as the stage is about to change. Basically it validates that before you move to 'stage 3', a field must be in the range of accepted values in 'stage 2'. A part of this logic was as follows:

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When we upgraded our non-production environments, this workflow stopped working as expected. I know that with the upgrade to 365, the business process flow structure and relationships had changed but what I found here was that the values being used to compare were the values before the change occurred and not after, even though the workflow setup hadnt changed:

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Quick fix in our setup was instead of checking if the current stage is 'stage 3', we changed the logic to see if it was 'stage 2' and looking to go to 'stage 3'
Hope that helps!