This has come up a few times on this list. Hans has stated a reluctance to make a change or add an option to discard the unused vertical space (see https://www.mail-archive.com/ntg-context@ntg.nl/msg21273.html from March 2007, and most recently https://www.mail-archive.com/ntg-context@ntg.nl/msg86872.html in January 2018).
Thank you for those pointers.
The workaround that Hans suggests in that last post , adding the flag 'high', almost resolves the issue in your example.
Right -- or just shifting the figure upward by one paragraph or so. In a long document like a book, pagebreaks need to be adjusted by hand at the end anyway to avoid long stretches of unused bottom space. Adjusting the figure placements slightly can become part of that process. -Sanjoy