Similar to QTBUG-54623. 

The problem is visible in tst_QFontDatabase::systemFixedFont():

FAIL! : tst_QFontDatabase::systemFixedFont() 'fdbSaysFixed' returned FALSE. ()
Loc: [/usr/src/debug/qtbase/6.2.1-r0/git/tests/auto/gui/text/qfontdatabase/tst_qfontdatabase.cpp(170)]

Debug print says:
QDEBUG : tst_QFontDatabase::systemFixedFont() system fixed font is QFont(monospace,9,-1,2,400,0,0,0,0,0,0,0,0,0,0,1) really fixed? false false

Strace reveails that the font actually loaded was MuseoSans-Medium.ttf:

readlink("/proc/self/exe", "/tst_qfontdatabase", 4095) = 18
openat(AT_FDCWD, "/usr/share/fonts/MuseoSans-Medium.ttf", O_RDONLY) = 10
fcntl(10, F_SETFD, FD_CLOEXEC) = 0
fstat(10, {st_mode=S_IFREG|0644, st_size=119592, ...}) = 0
mmap(NULL, 119592, PROT_READ, MAP_PRIVATE, 10, 0) = 0x7fc8240ac000
close(10) = 0
write(1, "QDEBUG : tst_QFontDatabase::syst"..., 143QDEBUG : tst_QFontDatabase::systemFixedFont() system fixed font is QFont(monospace,9,-1,2,400,0,0,0,0,0,0,0,0,0,0,1) really fixed? false false

In the file system there seem to be two monospace fonts, so maybe either one should have been chosen instead?
root@qemux86-64:/usr/share/fonts# ls -la|grep -i mono
rw-rr- 1 root root 438228 Dec 31 1969 NotoSansMono-Bold.ttf
rw-rr- 1 root root 426780 Dec 31 1969 NotoSansMono-Regular.ttf