It seems to be considered a good practice to use end-to-end HTTPS, but I'm not entirely sure why.
Modern security thinking is that you don't consider your own network / datacenter as more trusted (than your WAN or the regular internet).
Traditionally one would allow for more relaxed security standards in the datacenter, within the "secure" perimeter of your own network. Both internal systems and users would be trusted, implicitly expected to be secure and never abusive or malicious. One only added for example TLS for connections crossing the perimeter and borders of your "secure" internal network.
Your current configuration of terminating TLS on the load balancer and using plain HTTP for the communication between load balancers and the back-end servers fits in that traditional world view and security concept.
Nowadays the increasingly more prevalent security concept is one of "zero trust" and "encryption everywhere", which abandons the concept of a secure and trusted internal networks/systems/users and applies the same rigorous level of security everywhere, regardless.
In a cloud environment you don't have your own physical network but share resources with potentially all other customers of the cloud provider.
You hope that the cloud provider isn't malicious and can be trusted to not eavesdrop on your communication. You hope and expect that the logical separation of your (internal) traffic is robust and your data can't be intercepted by those other customers either, but you have much fewer guarantees there.
You control those risks by not implicitly trusting the security of the (virtual) internal network and rigorously applying TLS encryption, for the traffic from the load balancer to your application server, for the applications accessing the database, etc. etc.
As a real world analogue:
Your home is in a gated community, protected by a security fence and guards at the gate. Maybe you even have a owners association that vets potential new residents and only allow "the right kind of people" to become your new neighbours.
Would you need to lock your front door at night and when you leave your home?
In traditional security thinking: no you wouldn't need to lock your doors at night because you're safe in your gated community and all the "bad people" will be kept out by the fence and security guards.
And your neighbours won't be nosy gossips that will simply let themselves into your home.
In modern security thinking: yes of course you do lock your front door, because when (no if about it) the fence is breached and/or the guards fail to keep "a bad person" out, then that bad person can't simply enter every single home, but will still need to pick locks, or force entry, which both delays their unwanted access, increases the odds of being detected and reduces the number of house they can burglarise.
