RB433 MTU change after upgrade to 6.27 Bug ???

Hi All ,
after updating the rb to 6.27 , the mtu was reduced by 4 bytes .
Someone can help me , because I need to transit in 1526 MTU for MPLS+VPLS .
See attached photos.
Best Regards Giulio .
mtu.PNG
mtu-5.26.PNG
mtu-6.27.PNG

Yes same problem!!

Lost 4 bytes on the RB493AH!!

Was so confused on what happened!