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Dell Latitude 5420 / 5520 S3 Suspend (deep sleep) not working on Ubuntu 21.04


I just purchased the latest Latitude 5420 and I cannot get suspend to RAM working. I use Ubuntu 21.04 with kernel 5.11. dmesg shows there is no S3 supported:

# dmesg | grep ACPI | grep support
[ 1.423483] ACPI: (supports S0 S4 S5)

In BIOS I have Sleep S3 specifically unblocked (I also tried to change it to ON):


I also tried to add mem_sleep_default=deep to GRUB kernel boot options and also to force via command line, but mem_sleep file is not editable:

$ cat /sys/power/mem_sleep
$ echo deep | sudo tee /sys/power/mem_sleep
tee: /sys/power/mem_sleep: Invalid argument

When suspending (via systemctl suspend or from GNOME menu -> Suspend) I see it goes to s2idle always instead:

[ 1144.616225] PM: suspend entry (s2idle)

I tried everything I could find on the Internet but I cannot get it to go to deep sleep.

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