Hi Simon,
On your "detail_KD_TASK.htmpl" form you should have the following:
<PDM_MACRO name=dtlCheckbox hdr="Keep Published Version Available" attr="SWITCH1">
In your 12.6 environment this form may be in your mods directory if any changes were made to it.
Can you compare this line between the two environments?
I just did a quick test and set:
attr="SWITCH0"
This unchecked the Keep Published Version Available checkbox by default.
Thanks,
Scott