Disable WSL support for Win32 applications:
sudo bash -c "echo 0 > /proc/sys/fs/binfmt_misc/status"
Enable WSL support for Win32 applications:
sudo bash -c "echo 1 > /proc/sys/fs/binfmt_misc/status"
Is it done in the Ubuntu WSL shell, ...
This is expected.
which only searches for binaries in your $PATH variable (e.g. it needs to be placed in /usr/bin or /usr/local/bin for that). If you didn't put bitcoind there, you'll need to invoke it using the pathname (e.g. ./src/bitcoind in the directory that holds it).