CSV API does not always return CSV file

Example dataset: https://www.openml.org/api/v1/json/data/397

When I try to download CSV: https://www.openml.org/data/v1/get_csv/52509

One data row looks like:

{11 7,16 3,205 1,258 2,264 2,272 2,1585 1,1691 1,1939 1,1955 4,1958 1,1966 2,2024 1,2034 1,2047 1,2085 1,2127 1,2149 3,2332 2,2342 1,2370 1,2462 3,2558 1,2594 2,2603 1,2616 1,2623 1,2647 1,2653 2,2695 2,2857 1,3016 1,3046 2,3071 1,3104 1,3125 1,3130 2,3132 2,3154 1,3208 3,3224 3,3262 2,3265 2,3270 1,3272 1,3424 1,3470 1,3483 1,3495 1,3501 1,3561 1,3564 2,3584 1,3621 1,3754 3,3770 1,3787 1,3790 1,3816 2,3904 1,3905 1,3961 1,4122 1,4133 1,4136 1,4164 1,4236 1,4238 1,4239 1,4253 1,4257 1,4267 1,4288 1,4323 1,4331 1,4339 1,4473 4,4488 1,4543 1,4590 2,4606 1,4607 1,4744 1,4758 2,4816 1,4871 1,4996 1,5009 1,5041 1,5044 1,5049 2,5093 1,5114 1,5344 1,5350 1,5366 1,5438 3,5506 1,5507 1,5509 1,5521 4,5522 2,5523 1,5524 1,5525 1,5526 2,5535 1,5536 1,5537 1,5571 1,5578 1,5580 3,5609 2,5804 78}

I think the problem is that original dataset is sparse. It seems the converter to CSV files for sparse datasets does not work correctly?

Reproduce the problem with dataset 397 and the CSV endpoint for data 52509, then trace the sparse-dataset conversion path. Compare the returned content with the source row shown in the report; done means the API returns a valid CSV representation for sparse datasets.