vnstat data usage reporting inaccurate

[z0rk]

OPNsense 23.7.12_5-amd64

I have several interfaces but only WLAN WAN is selected in vnstat for usage reporting. It's consistently off by hundreds of GBs. My ISP enforces a data cap of ~1200GB, last month vnstat reported ~1790GB usage. I did not exceed my ISP's data cap.
Any suggestions? Thank you