Output Container lock button disappears

[pnharris02]

I'm fairly new to LosslessCut - great program!
I struggled to find a way to set a default output container.
Apparently there was meant to be a lock button next to the dropdown, but even in advanced view, my copy (the current version 3.64.0.0) didn't have this button. After lots of experimenting around, with suggested CLI options, the settings.json file and toggling advance off and then on again, I did manage to get the lock to appear. and a "outFormatLocked": "matroska" line was written to my settings.json file. This stuck for a while, but on a later session, out of curiosity I toggled the lock off, and the lock disappeared and wouldn't come back. I had to manually edit the config.json file and put the required line back in again.
There is no corruption of my settings file as I kept copies of the original, and even deleted it and let the program create a new one at one point.
What is happening here?