Switch Platform happens when Target Platform changed before Switch Platform button press

I assume its a setting somewhere but the issue is when we change target platform on 1 specific project in particular Unity automatically starts the process of switch e.g. before we click the Switch Platform button.

This is a massive waste of time as the project is big and the default CPU is 32 where we want 64 or Universal (Nix/Mac) so when we switch we have to wait for 32 bit to process then change to 64 wait again, etc. to build for all 3 platforms manually can take hours ... we have resorted to using a script but would like to know what setting it is that does this.

This for not is not the behaviour we see on other projects so we are sure its a setting we ticked somewhere but cant find to correct, e.g. properly switch doesn't occur till Switch Platform is pressed.