Thanks to Igor pointing me in the right direction after an email I was able to stumble through fixing this.
The bad:
Kestrel's support docs for the Drop are incomplete, out of date and contradictory.
The Drop manual doesn't cover the firmware update process and there are now two Link apps; Legacy and "new".
My Drop had firmware 1.53, after finally finding the firmware release notes I see this:
So given you have to use Link to update the firmware and that requires connecting to the Drop, would seem I have to do the upgrades in the Legacy app. Turns out NOT true.
Attempting the firmware update in the Legacy app appeared to go through the motions and I was then greeted by firmware version 1.50 so yet another version older than "Initial Release" that isn't documented. Attempting to do the upgrade again in Legacy the version didn't change.
At this point with nothing to lose installed the "new" app which DID connect to my now firmware 1.50 Drop D3.
The "new" app is busier but much more clear on how to upgrade the firmware and progress, it also has plenty of warnings which should be followed. Since I was now on 1.50 it took 3 update cycles to get it on 1.55 and a BTLE profile update as well.
So the real process is:
Use the Legacy app if you wish to save any of your historical logs off the Drop first. Once the update is done the Legacy app will no longer work and things can get wiped.
Put a fresh battery in the Drop. This process is intensive and will drain it, a used battery may cause the update to fail.
Make sure your phone is charged or on USB, the app needs to stay open and my working upgrades took well over 20 minutes.
Install the "new" Kestrel Link app it now has a white and blue icon.
Know your conditions.
play.google.com
Grant the permissions it asks for (Bluetooth, location, network and storage if it doesn't ask).
Make sure Bluetooth and Location are enabled on the phone.
Open the "new" Link app and Connect your Drop>Devices>Device Settings
Click Check for Firmware Updates on the next screen and read what it tells you, it can take several updates to finish the process.
After all was done, no more wild DA reading in Strelok Pro