Lightsail instance goes down without any CPU surge

Hello.

Lightsail has something called burst capacity.
Even if the baseline throughput set for each Lightsail specification is exceeded, it temporarily outperforms the performance and withstands the load.
With the specs of Lightsail you are using, bursting occurs when the CPU usage exceeds 10%.
When you burst, you use something called CPU credits.
If this CPU credit is used up, the performance will drop to the baseline throughput and the website may go down.
https://docs.aws.amazon.com/lightsail/latest/userguide/amazon-lightsail-viewing-instance-burst-capacity.html

So please check the metrics to see if Lightsail's CPU credits are 0.
If you can use SSH in other ways, please check the OS system logs to see if any errors have occurred.