'Impute missing values' does not work

I try to do a simple median or mode imputation on a column with integer values with a 'Impute missing values' step within a visual prepare recipe. The script output shows the column with correctly imputed values. However, after I run the script, this transformation step seem to have no effect although the other steps in this prepare recipe were computed correctly. The recipe is computed via the Spark engine, as this transformation step does not work with the normal DSS engine. The job does not show any errors. In the logs, I can also see, that this transformation step is at least part of the job definition:

[2021/05/26-17:09:56.148] [null-err-55] [INFO] [dku.utils]  -     {
[2021/05/26-17:09:56.148] [null-err-55] [INFO] [dku.utils]  -       "type": "FillEmptyWithComputedValue",
[2021/05/26-17:09:56.149] [null-err-55] [INFO] [dku.utils]  -       "params": {
[2021/05/26-17:09:56.149] [null-err-55] [INFO] [dku.utils]  -         "mode": "MEDIAN",
[2021/05/26-17:09:56.149] [null-err-55] [INFO] [dku.utils]  -         "appliesTo": "COLUMNS",
[2021/05/26-17:09:56.150] [null-err-55] [INFO] [dku.utils]  -         "columns": [
[2021/05/26-17:09:56.150] [null-err-55] [INFO] [dku.utils]  -           "XXX",
[2021/05/26-17:09:56.150] [null-err-55] [INFO] [dku.utils]  -           "YYY"
[2021/05/26-17:09:56.150] [null-err-55] [INFO] [dku.utils]  -         ]
[2021/05/26-17:09:56.151] [null-err-55] [INFO] [dku.utils]  -       },
[2021/05/26-17:09:56.151] [null-err-55] [INFO] [dku.utils]  -       "metaType": "PROCESSOR",
[2021/05/26-17:09:56.151] [null-err-55] [INFO] [dku.utils]  -       "preview": false,
[2021/05/26-17:09:56.152] [null-err-55] [INFO] [dku.utils]  -       "disabled": false,
[2021/05/26-17:09:56.152] [null-err-55] [INFO] [dku.utils]  -       "comment": "Impute XXX and YYY",
[2021/05/26-17:09:56.153] [null-err-55] [INFO] [dku.utils]  -       "alwaysShowComment": true
[2021/05/26-17:09:56.153] [null-err-55] [INFO] [dku.utils]  -     }

I'm working with DSS 8.0.2.

Any idea, why this is not working?