Suspend not working: shutting down instead

You can try this:

1. Edit the file /etc/default/grub (as root)

2. Change the line with:

   GRUB_CMDLINE_LINUX=""
   

   to:

   GRUB_CMDLINE_LINUX="acpi_sleep=nonvs"
   

3. run update-grub (as root)

Above taken from Suspend fails (reboot on resume) and no hibernate option [closed] The answer is from 2011 and GRUB_CMDLINE_LINUX may not work the same in which case modify the GRUB_CMDLINE_LINUX_DEFAULT line instead.

These other unanswered questions are similar to yours:

• Ubuntu shuts down instead of suspending
• Ubuntu shuts down on suspend?
• Dell XPS13 shuts down instead of suspending when on battery power - Gnome 16.04

I had the same issue with my Dell latitude e7270 and Ubuntu 16.04 LTS. My problem was solved when updating my kernel following the instructions here

This probably means it is an Ubuntu issue and I guess updating your kernel might do the job.