Oracle 12.2 client session PGA eating up all free memory on server

Just upgraded an application database from 12.1 to 12.2 on Oracle Linux 6, and now we're running out of memory on the server. The application servers are very inefficient (in my mind), and need 1000 connections to work. Yeah, wow. Anyway, under 12.2 we're seeing in v$process that each connection is getting up to 30MB of PGA memory allocated and it appears it is never getting freed up. So after using up all 20GB+ of free memory, everything starts dying from lack of memory and out of swap.

Specifically setting sga_target and pga_aggregate_target separately. pga_aggregate_target_limit is set to 0 because having the server run out of memory or having client connections killed mid-stream due to too much pga use are equivalent in that the app doesn't work.