I run "discard 1:1 previews" command and then I extract the preview with the Adobe script - ExtractPreviews.Lua
The script extracts 1:1 preview even after running the "discard 1:1 preview" command.
So ... when does Lr discard 1:1 previews?
Does "discard 1:1 previews" command not work?
There used to be a bug that prevented deleting 1:1 previews if your Standard Preview size is set to 1/2 the size of the image file long edge or larger.
What is your Standard Preview size and image file resolution? Keep in mind you may need to close LR and then restart it for the 1:1 Preview files to be deleted.
There used to be a bug
I think it's a deliberate design decision. If the 1:1 preview is "close" in size to the chosen standard preview size, LR will use the 1:1 preview for both purposes (1:1 and standard size).