Small workaround to show thumbnail before loading actual image with Coil.

CoilWithPlaceholder.kt

        var isOriginalImageLoaded = false

        // Thumbnail Request
        val thumbRequest = ImageRequest.Builder(ctx)
            .data(thumbUrl)
            .target(
                onSuccess = { result ->
                    // If highRes image is not loaded yet,
                    // show the thumbnail
                    if (!isOriginalImageLoaded) {
                        binding.headerImage.setImageDrawable(result)
                    } 
                }
            )
            .build()

        imageLoader.enqueue(thumbRequest)
        
         val highResRequest = ImageRequest.Builder(ctx)
            .data(highResUrl)
            .target(
                onSuccess = { result ->
                    isOriginalImageLoaded = true
                    binding.headerImage.setImageDrawable(result)
                },
                onError = { onFailed() }
            )
            .build()
            
        imageLoader.enqueue(highResRequest)

Thanks for checking and improving it :)