Transformer Voltage Inconsistent!

I installed a new Doorbell Pro 2 several weeks ago and it has performed reliably. I am using the Ring Plug-In Adapter (2nd generation) which is connected directly to the doorbell wiring. When I check the voltage with a multimeter at the doorbell it consistently reads 18 volts. Never changes, no matter how often or when I take a reading. The app, however, reports the Transformer Voltage in the Device Health tab as being all over the place from 15V (Poor) to 17V (Good). Either my multimeter is unreliable or the device's reading is unreliable? The voltage seems to read differently throughout the day and night when I check it from the app but the meter always reads 18 volts. Nothing is physically changing that would cause the fluctuation. At the moment it is 15V and poor. I suspect at the next check it will show 16V or 17V and good once again. This has been the case for days. At this point I am completely baffled.