Why is my stress score off the charts yet heart rate reads way below actual

From https://support.garmin.com/en-US/?faq=WT9BmhjacO4ZpxbCc0EKn9:

"the device uses heart rate data to determine the interval between each heart beat. The variable length of time in between each heart beat is fast regulated by the body's autonomic nervous system. The less variability between beats equals higher stress levels, whereas the increase in variability indicates less stress."

And from https://images-eu.ssl-images-amazon.com/images/I/A1c0MzlpaBS.pdf

The optical heart rate sensor on your Garmin watch uses light to penetrate your skin and estimate your heart rate. This allows you to get heart rate information without the need for an additional sensor such as a heart rate strap. To ensure best possible accuracy, follow the below guidelines.

• Make sure the watch is snug tight to the point it cannot move up and down your wrist. A little tighter when working out and a little looser when not.
• Wear the watch on outside of wrist, away from wrist bone
• Avoid wearing the watch over dark tattoos
• Check to see if the watch has locked on to your heart rate prior to starting your activity. You can confirm this by looking at your heart rate widget o Flashing heart symbol: Watch is looking for your heart rate o Solid heart symbol: Watch is locked onto your heart rate
• Ensure green lights (LED's) are functioning

Also, further in that document:

"Skin tone has an effect as the melanin in the skin absorbs some of the light. Garmin designs our watches to work on all skin tones. For higher Fitzpatrick classifications, more melanin is present in the skin. The sensor may have to work harder to find pulse which can require slightly more battery power

Tattoos absorb much of the light from the optical heart rate sensor which can cause major problems with accurately detecting heart rate. The sensor needs to be on an area free of tattoos."

In brief: the tattoos may well be the reason of the difference between the HR strap and the watch. The strap uses electrical signals to detect the HR, so even if you had tatoos over the chest too, it would still work well. And in the moment the HR detection of the watch does not work properly, for example skipping some beats, it will influence the HR Variability measuring and thus the stress detection, of course, too.

If you have no tattoos on your ankles, try attaching the watch there, just to see whether it then detects the HR correctly.