Please, try snapshot image as the fix was already pushed to master branch. Just make sure to use "sysupgrade -n" (without configuration preserved) and install LuCi manually after flash (snapshot images don't include LuCi).
I installed the latest snapshot from 10th July and I did reset the configuration to defaut in the process as you told me to. Unfortunately I was not able to install LuCi. I got error message "No space left on device".
"swconfig dev eth0 show" output is still correct.
Seems I need to compile an image by myself or wait for a release in order to test LuCi on this device.
I managed to make an image with LuCi on it with the image builder from snapshot.
For the record this is how I built it:
make image PROFILE=tl-wr740n-v1 PACKAGES="luci luci-app-firewall luci-base luci-lib-ip luci-mod-admin-full luci-proto-ppp luci-theme-bootstrap uhttpd uhttpd-mod-ubus"
I copied the sysupgrade image to /tmp on the device then I ran:
sysupgrade -n lede-ar71xx-generic-tl-wr740n-v1-squashfs-sysupgrade.bin
It was successful and I could login with ssh and browser.
On this image the port mappings are correct (swconfig and LuCi).