I was running KDE Neon on ThinkBook 15 G2 and had deep sleep working after adding mem_sleep_default=deep to GRUB_CMDLINE. It worked for a while until it didn't. I didn't do anything other than running regulat updates. Since couple weeks back, when going to sleep, it shows BIOS Recovery progress bar or something and restarts.
I switched to Debian and the behavior is the same. S2 sleep is next to useless as it drains something like 10% battery / hour, and the lap top is warm to touch.
have you updated EFI/BIOS recently? maybe S3 sleep is not supported on your system anymore and instead you get suspend-to-idle as S0ix (Modern Standby) notoriously shitty under linux. sometimes you can flip it back in EFI/BIOS