Lightsail instance becomes inaccessible periodically until reboot, but instance keeps running

For the last few months, we are periodically facing an issue where our Lightsail server becomes inaccessible (not pingable, can't be access through either https or ssh), but the instance itself does keep running and the logs on the instance show as much.

Also, even though the server isn't accessible, the server itself is still able to access the network. When we reboot the server though the AWS Console, we can access the server again.

The issue seems to occur randomly. Sometimes it's months before it happens again, at other times it happens again after a few days. It seems to be occurring more lately though.

I'm at a loss at what the cause of the problem is. Any suggestions or advice to help debug this problem is greatly appreciated.