The P60, P70, and P80 performance tiers can only be used by disks that are larger than 4,096 GiB. A disk’s performance tier can be downgraded only once every 12 hours. The system does not return Performance Tier for disks created before June 2020. You can take advantage of Performance Tier for an older disk by updating it with the baseline Tier.
https://docs.microsoft.com/en-us/azure/virtual-machines/disks-performance-tiers