and thus catching /all/ explicit requests for "MS Sans Serif" and "Microsoft Sans Serif" ...which is what WinForms specifically requests.
Thus the explicit "grabbing" of the pseudo generic font family name causes everything to be sourced from that font, which has no coverage other than for basic (non-accented Latin) and for Thai.
This has eventually been traced back to a specific font package (providing Thai fonts) having a slightly too-broad match in:
/etc/ fonts/conf. avail/89- ttf-thai- tlwg-synthetic. conf
and thus catching /all/ explicit requests for "MS Sans Serif" and "Microsoft Sans Serif" ...which is what WinForms specifically requests.
Thus the explicit "grabbing" of the pseudo generic font family name causes everything to be sourced from that font, which has no coverage other than for basic (non-accented Latin) and for Thai.