Why does ALB go down?

I don't think we (as participants on re:Post) can answer this question as we don't have access to your environment. Our support team is much better placed to help here so please raise a case with them if your ALBs are going down.

To probe a little deeper though:

You mention "pings the AWS ALB IPs to keep the connection active" - why? You don't mention the protocol you're using - are you trying to keep a websockets connection open permanently? I'd note that networks are (despite all appearances) unreliable (in some way) - there are always going to be cases where a connection will drop and it's a good idea to build applications so that they automatically reconnect.

If you're redirecting users (presumably a browser session) to the ALB after login then each browser will be establishing a new connection so keeping the connection alive isn't necessary.

Note that the ALB IP addresses may change from time to time. The DNS name won't; and the DNS entry will always be up to date. ALBs might be replaced during scaling events or if the underlying hardware has an issue. So don't rely on the IP addresses being static - that isn't the case here. Always use the DNS name to connect. An event where the ALB changes IP address may also cause your connection to drop.

This is all general advice because the specifics aren't in your question but I hope it helps.