I downloaded and installed Windows versions of Flickrdownloadr (version 2.5.1.7) and ExifTool (version 11.08).

I uploaded an image (with no specific exif data) to flickr and added it to an album.

I changed the title and description to known values (in Flickr). It appears Flickr does not add these values to the image file exif data - it adds it to its own metatdata for the image - which Flickrdownloadr picks up and writes to the JSON file for each image.

I downloaded the album using Flickrdownloadr, which downloaded the image and its sidecar JSON file (containing the two values for title and description).

I then followed this answer on stack overflow to write the values from the JSON file into the exif data in the image file using:-

exiftool -json=x.jpg.json x.jpg

(where x.jpg is the name of the image file)

I then uploaded the image back to Flickr - and (now that the Flickr values were burned into the image file) the title and description received the correct values from the original album in Flickr.

Applying the sidecar JSON files to the images can, in Windows, be automated using:-

for %1 in (*.jpg) do ..\exiftool -json=%1.json %1

(where exiftool.exe was installed in the parent folder of the current/download folder)

You'll need to figure out how to do that for any other OS you may be using.

In Linux/bash this will achieve the for loop:

for jpg in *.jpg; do exiftool -json=$jpg.json $jpg; done

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The JSON file created by Flickrdownloadr has this format:-

{"Tags":"kitten","Description":"flickr description","Title":"flickr title"}