Another update.
The turrets (2x 2CD2385) were in fact using the firmware version 5.5.82 (Build 181211), dated "2018-12-27"
I rolled back the firmware to 5.5.80 and the cameras now appear to be working correctly!
Conclusion is do not use 5.5.82 - G1 platform (H.265 DS-2CD2XX5,2XX3,3XX3,3XX5,XM67X6)