Hi,
this is a limitation of TTreeReader, it does not support 2D arrays. As RDataFrame uses TTreeReader under the hood for I/O, it inherits this limitation. RDataFrame does not support multi-dimensional arrays.
See e.g. ROOT data frame - unsupported leaf value . The relevant JIRA ticket is [ROOT-9509] [DF] Add proper support for multidimensional arrays - SFTJIRA . CC: @Axel .
Cheers,
Enrico