Why is Linux only recognizing 2GB of my 16GB RAM?

I'm experiencing a frustrating issue where my Linux system can only detect 2GB of RAM, even though I have a total of 16GB installed. I'm dual booting Linux on an SD card alongside Windows, which recognizes all 16GB without a hitch. I'm using an HP ZBook Studio G5, and I initially set up dual booting about 8 months ago without any problems. Recently, however, I noticed Linux would sometimes recognize 15GB after a complete reinstallation, but it eventually dropped back to 2GB after a few days. I ran the 'free -h' command and confirmed that it only shows 2GB total memory (including swap). I also found a bunch of ACPI errors, which some sources suggest could be the problem. They recommend rolling back my BIOS version, but I'm looking for advice before taking that step. I've already tried various fixes, including a GRUB memmap override, ensuring I'm booting in UEFI mode, and disabling Secure Boot in the BIOS. Any insights on what might be going wrong?