I just figured out it is not related to the HiDPI detection, not directly at least. Ubuntu 18.04 comes with Gnome and in the display settings scaling is only provided in steps of 100%, 200%, and 300%. 200% is too large, 100% a bit too small, that's why I went for 100% and 'Large Text: enabled' in 'Universal Access'. It's the 'Large Text: enabled' setting that makes Pd menu and iemgui font sizes larger. When I disable that, your test patch looks consistent with your Debian Buster, Windows and macOS version.
XFCE is also based on GTK+, maybe you find a similar setting in your control panel?
Roman, thanks a bunch, you really pointed me in the right direction !
the bad guy was "Custom DPI Setting" in the "Settings/Appearance/Fonts" Section of XFCE. This setting overrides the monitor resolution with a custom DPI size that seemingly also distorts the spacing of fonts, at least for PD's fonts.
Disabled it ... now the test patch looks the way it should !
Case (almost) closed ! (at least for me, at least for now)
This probably should be mentioned in PD's README, so people know where to look for possible culprits concerning font issues.
best & mille grazie again !
Oliver