whereas, to venture a conjecture,
this Jaan (van Brugge), lord of Gruythuyze - of the family of Grimberghe lords
http://genealogics.org/getperson.php?personID=I00085954&tree=LEO
who was adult, to himself marry in 1389,
would suit to be born of Isabella's second marriage (which took place after 1365)
http://genealogics.org/getperson.php?personID=I00002064&tree=LEO
so, what is known in near-contemporary materials about who would have been mother of this Jaan ?
and what is known of who inherited possessions of Isabella de Looz, heiress of Agimont and Ayschove ?