test_tri_diff_mat in 3D crashes eventually

Running this command:

while pycl test_grudge.py 'test_tri_diff_mat(cl._csc, 3)'; do true; done

(Py3.7, POCL 6f31aa89) will crash.

faulthandler says:

Thread 0x00007ff935122740 (most recent call first):
  File "/home/andreask_work/src/pyopencl/pyopencl/__init__.py", line 1669 in enqueue_copy
  File "/home/andreask_work/src/pyopencl/pyopencl/array.py", line 627 in set
  File "/home/andreask_work/src/pyopencl/pyopencl/array.py", line 1958 in to_device
  File "<generated code>", line 213 in invoke_diff_loopy_kernel
  File "/home/andreask_work/src/pytools/pytools/py_codegen.py", line 199 in __call__
  File "/home/andreask_work/src/loopy/loopy/target/pyopencl_execution.py", line 355 in __call__
  File "/home/andreask_work/src/loopy/loopy/kernel/__init__.py", line 1408 in __call__
  File "/home/andreask_work/src/grudge/grudge/execution.py", line 397 in map_insn_diff_batch_assign
  File "/home/andreask_work/src/grudge/grudge/symbolic/compiler.py", line 529 in execute
  File "/home/andreask_work/src/grudge/grudge/execution.py", line 496 in __call__
  File "test_grudge.py", line 156 in test_tri_diff_mat
  File "<string>", line 1 in <module>
  File "test_grudge.py", line 585 in <module>

Gdb implicates _pocl_kernel_grudge_assign_0_workgroup.