This happened when the UDIM tiled image needed to be colormanaged, so

• when you set up the image as sRGB, then save as EXR/HDR/...
• other way around as well: when you set up the images as Linear then save as PNG/JPG/...

Reason being that for UDIM tiled images, image_save_single is called
multiple times [once for each tile] and everytime image_save_post will
fire the IMA_SIGNAL_COLORMANAGE signal which clears the cache if any of
the above two is the case. Without the cache, the next tiles cannot be
saved.

Now determine if the colorspace changed from image_save_single/image_save_post and only fire IMA_SIGNAL_COLORMANAGE once from BKE_image_save in the end.