This is a known regression, tracked as steamrt/tasks#1004 internally.
This is happening because the experimental steamrt3c Steam client is now running inside a container, with a different /usr, so it needs to follow different logic to find /usr/share/applications/steam.desktop and copy it to the autostart directory.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
Can`t enable "Run Steam when my computer starts"
Steps for reproducing this issue: