An update: BIOS 0704 fixes the issue for me. It's too bad that it hadn't been mentioned in this thread yet -- it's been available in the forums for 11 days.
"(Updated some vrm porting, dram compatibility)"
I take this back. I'm now able to complete stress tests at 4000 16-19-19-39 with my Ballistix 3600 16-18-18-38 32 GB kit, but the system randomly crashes and powers off when performing light tasks like opening new tabs in Chrome -- this is happening 100% with light workloads using the XMP profile, and doesn't happen when one of the channels is disabled. Dropping to 2133 MHz also resolves the issue.
Should I give up and install the replacement Z490-I that's sitting on my desk?
Another update! Matching Min CPU Cache Ratio to Max CPU Cache Ratio (48 in my case) appears to have fixed the issue with light workloads. Possibly a useful datapoint for the engineers at ASUS?
Sorry to hear that. Did you try manually setting VCCIO and SA voltages to 1.1V as @morello1n stated?