French Door Frame Flexing - Cracking Mortar

Hi, I have had some great 1970x2150 french doors fitted. The door gap was 2010 so a knock on was added each side. 4 screw fasteners were used each side and 2 on the bottom sill - none on the top due to the lintel. The opening door frame flexed slightly which initially hairline cracked the internal plaster and also dislodged the outer mortar fillet which was used to finish the outside. It became hollow sounding and separation from the plastic frame could be seen. The mortar was removed and gap filled with foam (initially the gap was NOT filed at all which concerned me) and re-mortared. The frame is more solid but the remaining slight movement has caused the mortar to become hollow sounding around the top hinge, presumably where there is most weight and leverage. I suspect the slight flex has loosened it again. My question is ... Should I expect a solid frame fixing ? And is foam adequate in the gap or do packers need to also be fitted with perhaps longer screw fasteners ? Or should I expect some flex in the frame and simply replace the mortar (which has a very appealing look) with a UPVC trim or wooden trim to allow the flex ? I don't want to ask something of the fitter again if I am being unreasonable where there is an alternate finish. The installation and doors look great and the flex is minor. However the fact no gap filler was applied at all initially just makes me nervous that something else could be done. Many thanks :-)