Partitioned mariadb table files(the .ibd ones) placed outside of data directory return back to data directory itself after altering the table in use
That's a problem with ALTER.  File a bug report if you like.

I consider RAID striping to be better than manually placing pieces of the database across physical volumes.

What kind of ALTER are you doing?  Adding an index is different than adding a column is different than fiddling with the partition structure.  There may be a workaround.