The duchy has the plague at least since mid 1423. Agnese del Maino is pregnant (twice) and Filippo Maria needs urgently a heir. Abbiategrasso is later the place for Agnese del Maino. As far I remember the dead child of Agnese had been born before Bianca Maria (* March 1425). Wrong ... she was born 1426, says wiki.
Klaus Schelle (? as I remember) makes the remark, that Filippo Maria started his more or less private life outside of public attention c 1421. The plague, the twice pregnant woman and the development of his solitairy life-style go all together. And Agnese stays in Abbiategrasso with his single daughter. Probably the second dead daughter causes, that Filippo Maria marries a second time in 1427. This was in December.
Interestingly John VIII of Byzanz divorced in 1426 and remarried 1427 in September. This case didn't work and the marriage of Filippo Maria also didn't work. And both had before the same woman Sophia, Filippo as a proposed husband, and John in the real normal manner.
Emperor Sigismund had also problems with the missing heir. King Wenzel dito.
I took a curious look ... I detected a theatre play and a text of 2016. We had great history articles in a German encyclopedia Ersch-Gruber in 19th century. Unluckily it was never finished and V as Visconti and S as Sforza weren't part of the finished letters. If one wants to become famous one better has a name with starts with A and not a name, which starts with Z.
Filippo Maria Visconti. Dramma storico
https://books.google.de/books?id=ENlVAA ... navlinks_s
Il ducato di Filippo Maria Visconti, 1412-1447. Economia, politica, cultura
Cengarle, Federica, Covini, Maria Nadia
Firenze University Press, 2016 - 374 Seiten
https://books.google.de/books?id=e649Cw ... navlinks_s