Busybox su leaves EUID unchanged

[erkska dev]

Hello.

Environment: aarch64 embedded device, Linux 5.4.70, Busybox v1.33.0
# cat /etc/passwd
root:!:0:0::/:/bin/sh
userx:!:203:203::/:/bin/sh

# id
uid=0(root) gid=0(root)
# su userx
# id
uid=203(userx) gid=203(userx) euid=0(root)

Is this expected behaviour? From source I can see it does setuid() which
should also modify euid. If not then what could be the issue?

Thanks!
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