I have no idea why it behaves like that in your case
There is no detailed description of the actual ingestion process... I know from experience what it does, but that's not the official description, it would be own :-) with its flaws!
That said, do you know if the customer is using the command "arslog" to do things? As you probably know the "arslog" can be modified, since this is a shell script. And maybe this one is doing more than what it should do.
And maybe because the application group are "old", it takes time for it to process it, since there are some backlogs. And when you have a new application group, there are no backlog, and therefore it goes quick.
Do you know if they are using some user exits?? From what you've shown us, I have the impression that no... but I might be wrong.