This came up in an install that I was just doing for Debian 10.10 media testing.
I hadn't seen this before and it would be disconcerting to other people, though it is a known bug, I think.
I was installing an image that had no network and no firmware. KDE failed to run and dropped me to a text mode prompt. This was because the Zotac SBC I'm using requires Radeon R600 firmware to work. There was a warning message on screen to that effect.
The way round this was to plug in a network cable and edit /etc/apt/sources.list.
Editing /etc/apt/sources.list was to add contrib and non-free to the appropriate lines to allow me to install firmware-linux-nonfree and firmware-misc-nonfree which includes the appropriate AMD firmware for the embedded Radeon chipset.
Since the machine hadn't been connected to a network at install time, I also needed to run a dhclient command to obtain a network lease and allow me to install the non-free metapackages over the network.
Result: success: a full KDE desktop. [The machine is an old Zotac SBC with embedded graphics hardware: AMD E350 - specifically, it requires firmware-amd-graphics and amd64-microcode].