What is the expected behavior for updating formula fields on the offline platform?
If there is a formula field on an object, Salesforce needs to run its normal save logic when the dependent fields are populated.
Therefore the changes on the dependent fields are synced up first with Autosync, then when Salesforce updates the formula field, the changes are pushed to the end user's data feed.
Offline devices retrieve information from the data feed automatically. It is expected that the user won't see the formula field value instantly. It takes a couple of seconds until the application updates the formula field from the data feed.
CRM Help Documentation: Using Autosync