Yeah, looks like a build issue somewhere, as this is happening across versions and targets, although I dug through failure logs and found nothing (https://downloads.openwrt.org/releases/ "faillogs*" down near the bottom).
I also see it on 22.03 and SNAPSHOT, x86 and mt7622...
Side note: libncurses is just an alias for libncurses6, you can see this in the "provides" section of package info, and in many instances you can interchange the name.
$ opkg info libncurses6
Package: libncurses6
Version: 6.4-2
Depends: libc, terminfo
Provides: libncursesw, libncurses, libncursesw6
Status: install ok installed
Section: libs
Architecture: x86_64
Size: 182507
Filename: libncurses6_6.4-2_x86_64.ipk
Description: Terminal handling library (Unicode)
Installed-Time: 1694241770
$ opkg install libncurses # Try to use the alias, and it works...
Package libncurses6 (6.3-2) installed in root is up to date.