UEFI ConOut with systemd-boot: attributes aren't working

I've got a couple of Traverse ten64 machines, which have serial consoles and are using U-Boot 2023.07-rc2 (a bit old, granted). I connect to their consoles using Minicom on a Debian machine.

If I stop the U-Boot autoboot process, the U-Boot menu itself is displayed as expected, with 'reverse' highlighting on the currently-selected item in the menu. If I continue the boot process, then systemd-boot displays its menu (using the UEFI ConOut interface), but there is no highlighting on the menu items. I've tried both VT102 and ANSI emulation modes in Minicom.

Can anyone suggest steps for troubleshooting this?