i'm not sure if i understand your question. I assume you're asking what issues i'm seeing when this MTU change occurs?
Because of the MTU change, i have TLS handshake problems (websites using https do not work, so pretty much the internet) when browsing the web on a clientdevice behind the firewall that gets the public IP bridged by the RUTX. I needed to lower the MTU/MSS-clamping setting on my firewall to be able to browse the internet properly.
I do not have these problems when using NAT mode.