Keep in mind that all the functions evaluated during form processing occur on the user’s device at the time the form is opened and as the user interacts with the form.
Further, the user’s device is using a copy of the data as of the last sync. Each user of a multi-user app is using their own copy. Only when the app syncs does the device copy update to reflect data synced by other users.
Given the above, using any formula to construct a unique ID value based on existing data is risky, since the formula cannot be aware of decisions made on other users’ devices until everyone has synced.
For instance, if two users sync simultaneously, so they have the same data set, then each adds a new row with an ID constructed using MAX(), each ID would be the same because each user would compute the same max value from the same data. When they then sync again, the data from the first to sync would be overwritten by the second.
Please see the attached. Sequential Keys help.appsheet.com