I agree. This was a huge jaw drop change that I found completely unnecessary. In several breakthrough maps, I have been run over by vehicles before I could get in the conflict zone and set mines in actual viable places to prevent the advancement. This was not a smart change. Defenders should be able to set up flanking countermeasures at any time in their owned spaces.
Offense should be able to set down mines in the places they have captured and is the current "HQ" zone. It may not be necessary but you should be able to.
This one was puzzling for sure.