Why does underscore for last output not work in worksheets uploaded from local SageMath to Cocalc?

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My commands are like this:

vector([x1,y1,z1])

r1 = _

And I get this message:

Error in lines 1-1
Traceback (most recent call last):
  File "/cocalc/lib/python2.7/site-packages/smc_sagews/sage_server.py", line 995, in execute
    exec compile(block+'\n', '', 'single') in namespace, locals
  File "", line 1, in <module>
NameError: name '_' is not defined

However, copying and pasting commands into a new worksheet make that feature work again. Why?

I have links to illustrate it, but my karma is insufficient to post them here.