Apple Watch SE (2nd generation) battery dying at 20%

It sounds like a faulty battery. No amount of resetting suggested by the pernicious AI bot above will fix a battery that dies at 20%.

It is suspicious having 100% health for more than 3 months.

You ought to reach out to Apple Support directly. You should be able to reach them here  Click here > Contact - Official Apple Support  or phone numbers  Click here > Contact Apple for support and service - Apple Support   or Click here > Genius Bar Reservation and Apple Support Options - Apple 

If your Apple Watch SE is experiencing unexpected battery drain and shutting down at a higher percentage than usual, there are several steps you can take to try and resolve the issue:

1. Restart your Apple Watch: Sometimes, a simple restart can help resolve software-related issues that may be causing abnormal battery behavior. To restart your Apple Watch, press and hold the side button until the "Power Off" slider appears. Slide it to power off the watch, then press and hold the side button again to turn it back on.
2. Update watchOS: Make sure your Apple Watch is running the latest watchOS version. Software updates often include bug fixes and optimizations that can improve battery performance. To update your watch, make sure your iPhone is connected to Wi-Fi and has sufficient charge. Then, open the Watch app on your iPhone, go to "General," and select "Software Update."
3. Check for Battery-Intensive Apps: Some third-party apps or features might be draining your Apple Watch's battery faster than usual. Monitor your app usage and check if any specific apps are consuming excessive power. Try uninstalling or limiting the use of these apps to see if it improves battery life.
4. Reset Settings: You can try resetting your Apple Watch settings to default. Note that this won't delete your data or apps, but it will reset your preferences. To do this, on your Apple Watch, go to Settings > General > Reset > Reset Settings.
5. Unpair and Pair Your Apple Watch: If the issue persists, unpair your Apple Watch from your iPhone and then pair it again. This process can help resolve software-related issues. Before unpairing, make sure to create a backup of your Apple Watch data. To unpair, open the Watch app on your iPhone, tap your watch at the top of the My Watch tab, and select "Unpair Apple Watch." After unpairing, follow the pairing process to set up your Apple Watch again.
6. Contact Apple Support: If none of the above steps resolve the problem, and your Apple Watch is still shutting down prematurely, it's possible that there may be a hardware issue. In this case, it's best to contact Apple Support or visit an Apple Store for further assistance and evaluation of your device.

Before taking any major steps, keep in mind that battery performance can vary depending on usage, environmental conditions, and other factors. However, if you notice a significant and consistent decline in battery life, it's essential to address the issue to ensure optimal performance of your Apple Watch.