HiDPI scaling breaks rendering of some (bitmap) fonts

If HiDPI display scaling is enabled, some (bitmap?) fonts, such as Terminus, are rendered with additional letter spacing. The size of the spacing depends on the scale factor (the bigger is the factor - the more space is added). It looks like instead of properly scaling glyphs, font rendering engine just adds empty space around them.

When Qt HiDPI scaling is disabled (i.e., when only Xorg HiDPI is used), all fonts are rendered fine. The same when QT_SCREEN_SCALE_FACTORS is unset.
 
Using KDE (5.12?) on recent Debian testing. The version of Xorg is 7.7.
 
P.S. I also tried to resolve the issue by "enforcing bitmaps" using FontConfig as described, e.g., here. No luck. So, this looks to be a Qt issue. Are there any other workaround?

Related issues: QTBUG-4045 and QTBUG-67500.