I’m afraid I must be dumb for not quite understanding this part when looking at the settings’ definitions:
composer_media_optimization_image_dimensions_resize_threshold: default 1920
Minimum image width to trigger client-side resize
composer_media_optimization_image_dimensions_resize_target: default 1920
Images wider than composer_media_optimization_image_dimensions_resize_threshold will be resized to this width.
Must be ≥ that threshold.
This part specifically:
Must be ≥ that threshold.
Shouldn’t it be ≤ instead? I don’t understand why it would resize to a higher width value than the threshold 
Also, in the same setting’s description, the first setting is misnamed:
composer_media_optimization_image_dimensions_resize_threshold
The right name is “Composer media optimization image resize dimensions threshold” (“resize dimensions”, not “dimensions resize”).