This is with firmware .144. Steps to reproduce:
- power on machine
- hit temperature, set both nozzles to 230 C
- before it gets to 230, hit menu -> change filament, then nozzle 2
- after a minute or so, it'll say it has a heat runaway and then reset. When it comes back nozzle 1 will be around 260.
So, seems that the printer will keep heating nozzle 1 with no limit while it is switched to nozzle 2. If you let it reach 230 before step 3, there won't be a runaway.