The following two lineages (as genealogics currently presents them) have a look that one might be a garbled version of the other, at least in part.
* 1 Gildolf II van Brugge
Heer van Gruuthuse
o
+ 2 Katharina van Brugge
# Gerhard van der Aa, Heer van Grimbergen, fl 1288
* 3 Johan I van Gruuthuse, Heer van Gruuthuse, Grimbergen & Pollaer d. Aft 1351
o Margareta de Ghistelles
+ 4 Johan II van Gruuthuse, Heer van Grimbergen en Pollaer, fl 1382
# NN
* 5 Johan III van Brugge, Heer van Gruuthuse - marriage in 1389
o Agnes de Mortagne d. 1438
+ 6 Johan IV van Brugge, Seigneur de la Gruythuyze d. Aft 4 Jul 1438 - marriage in 1416
is copied from:
Europ�ische Stammtafeln, J.A. Stargardt Verlag, Marburg, Schwennicke, Detlev (Ed.), vol: IX
http://genealogics.org/descend.php?personID=I00085688&tree=LEO
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* 1 Leon van der Aa
o
+ 2 Gerard van der Aa d. Aft 1297
#
* 3 Jan van der Aa d. Abt 1360
o
+ 4 Jan van der Aa, fl 1376
#
* 5 Jan van der Aa, Heer van Grimberghe, fl 1385
o Isabella de Looz, Heiress of Agimont and Ayschove
+ 6 Johanna van der Aa, heiress of Grimberghe - her second marriage allegedly in c1374, though this pretty much is a misfit compared with fl 1376 and fl 1386 'grandfather' and 'father'
is mostly copied from
Manuel d'Histoire, de Genealogie et de Chronologie des Tous les Etats du Globe Leiden, 1888-1893, Stokvis, A. M. H. J., vol III
(which is likelier to be a copy of a concoction)
http://genealogics.org/descend.php?personID=I00558883&tree=LEO
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So, before the 1374 Johanna needs to have been old enough to have married her first marriage.
This all means that it is pretty implausible that this Johanna were daughter of Isabella de Looz, heiress of Agimont and Ayschove
http://genealogics.org/getperson.php?personID=I00002064&tree=LEO
because said Isabella's first husband,
Jean de Walcourt, seems to deceased as late as in 1365.
There is simply not time enough for Johanna to born of Isabella's later (= obviously after 1365) marriage, and then be old enough to be herself widowed once already as early as in c1374.
So, does anything contemporary prove this Johanna to be that Isabella's daughter ?
And if proves, how then are explained or estimated these dating points?
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 ?