Try to use vips_thumbnail_buffer on heic(3024x4032) image with params:

size = VIPS_SIZE_DOWN
width = 1200
height = 1200

But the result has size equal to 240x320. I guess that it tries to generate thumb from heic preview(jpeg). Is this what you intended or is it a bug? If it should be, then how can I disable generation thumb from heic preview(jpeg).

Also i get thumb 240x320, if use params:

size = C.VIPS_SIZE_DOWN
width = C.VIPS_MAX_COORD
height = C.VIPS_MAX_COORD

But if i use size = C.VIPS_SIZE_BOTH, the result is thumb of requested size.

Sample