I enabled kms debugging with drm.debug=4 on the kernel command line. This enabled me to see that the relevant modes are being rejected with error 11 which is DC_EXCEED_DONGLE_CAP. Sure enough, I see that there's a dongle detected that's rated at 165MHz and my preferred modes are all above that.

However, if I switch the monitors around everything is fine, because I can use a lower resolution monitor with the dongle and get the 21:9 resolution on another output without a dongle.

The dongle is actually a passive DP to HDMI cable, and I need it because one of my outputs is DP and I don't have any monitors with DP inputs. I was told somewhere that I'd need to use the DP output to get the clock speeds needed for 21:9, but apparently that's not true. Clearly the cable can go above 165MHz since the 4.14 kernel can drive the display at the preferred resolution using it, but thankfully I don't need to concern myself with that now.