Either this is a bug or the feature is currently not supported. The landmass blueprint brush breaks after using the Landscape Resize tool with “Resize Mode” set to “Resample”. However, you can get the information back by resampling back to the same settings you had previously (ie. if you resampled from 63x63 quads Section Size to 127x127 quads, switch it back to 63x63 and hit apply and the brush will reappear).
On the other hand, if you make changes to the settings but set the “Resize Mode” to use “Expand” or “Clip” and then hit Apply, your landscape brush will be preserved. Unfortunately, these 2 other settings don’t really let you increase the “Overall Resolution” of the landscape much, they mostly affect the “Number of Components” and “Total Components” (you can compare what values will change based on your current settings under the “Original” and “New” columns of the “Change Component Size” dropdown when you activate the “Resize” tool).
