This was a known problem on this firmware and has been reported elsewhere in the forums if you search around and as you discovered, it’s fixed in 384.19.No matter what I put for upload/download bandwidth on a new RT-AX88U on Merlin 384.18, it will peg the upload speed at my ISP's maximum speed and ping plots go to hell. The download appears to be limited appropriately based on what is set for the download bandwidth. Everything appears to be limited appropriately if I use traditional QoS, and the download bandwidth is limited with adaptive, just not upload. I don't have any QoS mods installed yet.
I have been running 384.19 for a while on mine and it’s been very stable - the major changes are with the Open VPN implementation so if you don’t need that, I would say it’s quite stable.