I’m not sure but this might be related to the recent question: How to use arguments with hyphens in math mode.
The documentation shows that you can use a set command to do this:
#set math.mat(row-gap: 1em)
$
mat(1;2;3; delim:"[")
$
Which renders the same as what you posted using the argument gap.  This will affect all matrices after this unless you encase this part in its own scope:
Code wtih Scope
$
mat(1;2;3; delim:"[")
$
#[
  #set math.mat(row-gap: 1em)
  $
  mat(1;2;3; delim:"[")
  $
]
$
mat(1;2;3; delim:"[")
$
Output
