memtest86+ and memtest86 < 5.0 don't support UEFI nor DDR4, becoming obsolete in many ways. However Gentoo users have no chance of knowing it upon emerging them. At the very least, einfo in pkg_postinst should tell it.
 See the "history" page in memtest86.com, https://www.memtest86.com/history.htm, in particular the section "MemTest86 and MemTest86+" and the first few sentences in the next section.
BTW bug#660504 is a bump request to recent memtest86.
Thanks Gentoo developers. Best regards.