The matrix is being created and written correctly to the disk. The matrix can also be used in subsequent calculations.
However, there appears to be a bug in reading the header of the matrix when displaying in the data editor. This will be repaired in the next update (11.1) due shortly.
In the meantime you can open it in notepad as the file is simply text.
To write out a large matrix and override the default display, use a FORMAT statement. This example creates a 20 by 20 identity matrix and writes it to a file. See the SHAZAM Manual for more examples.
* Create an identity matrix 20x20
matrix x = iden(20)
* Define how to output
* i.e. 20 columns of floating point of up to 8 digits with 4 decimal places
* Write the matrix x to the current path and use the previous format statement
write(xmatrix.mtx) x / format