NCC.GEN.105 Crew responsibilities (NCO.SPEC.115 is similar)
(a)The crew member shall be responsible for the proper execution of his/her duties that are:
(1)related to the safety of the aircraft and its occupants; and
(2)specified in the instructions and procedures in the operations manual.
(b)During critical phases of flight or whenever deemed necessary by the pilot-in-command in the interest of safety, the crew member shall be seated at his/her assigned station and shall not perform any activities other than those required for the safe operation of the aircraft.
(c)During flight, the flight crew member shall keep his/her safety belt fastened while at his/her station.
(d)During flight, at least one qualified flight crew member shall remain at the controls of the aircraft at all times.
From Regulation (EU) No 965/2012.
Reading between the lines, as long as a qualified flight crew member remains at the controls, other persons would be able to swap seats whenever deemed necessary by the PIC, including the PIC. The ANO 2016 article is applicable to non-EASA aircraft and is more limiting. So it looks like it depends on the aircraft at this point. But perhaps I should have dug a bit deeper.