Kernel no serial console output if started through U-Boot

Question

How does U-Boot change how the kernel is communicating through UART?

I'm trying to add U-Boot to our project. We build a custom Linux 64 bit kernel with initRAM linked into the kernel.

I have access to the U-Boot shell but when I'm trying to boot the uncompressed Image

mmc dev 0
fatload mmc 0:1 ${kernel_addr_r} Image
fatload mmc 0:1 ${fdt_addr} bcm2710-rpi-cm3.dtb
setenv bootargs console=serial0,115200 console=tty1
booti ${kernel_addr_r} - ${fdt_addr}

I get no kernel output after Starting kernel ...

What confuses me is that if I start the kernel directly through config.txt I do get the expected behavior.

The kernel command line is in both cases console=serial0,115200 console=tty1 Set either by cmdline.txt or through boot.scr.

Is U-Boot changing somehow the routing of the UART's? Why would the kernel have different behavior solely by starting it through U-Boot? Shouldn't U-Boot be completely out of the picture after the kernel is started, thus have no impact on the kernel output?