Thanks Oliver for the prompt and helpful response. It belatedly occurred to me to check that the camera and lens I was using would otherwise perform properly - and it turns out that the camera and lens combo (Canon 5D Mk IV with Sigma 1:1.4 DG 50mm) has communication issues between lens and camera; ...

Partially answering myself; I did update the firmware, which made no difference. However what did change the behaviour was changing the algorithm; the problem I first describe came from the EV 24h algorithm. When I changed to the LRT Sunset beta (yes, it was sunset), it respected the start value and...

Wonder if anyone knows what might be going on. I am trying a day/night timelapse, and I set my parameters for Day Tv to 1/640, and also the same value for Start Tv. In a number of attempts, whatever i set as the Day limit, and the Start Tv values, within a few frames the Unleashed has rapidly droppe...