CI: non-determinstic rare fullworkflow failures

Sometimes I'm seeing:

** Completion data **
Produced papers: 20
Scanned papers: 18 (currently)
Completed papers: 2-16, 18-20
Identified papers: 2-16, 18-20
Totalled papers: 2-16, 18-20
Number of papers with 0 questions marked = 0. Tests numbers = 
Number of papers with 1 questions marked = 0. Tests numbers = 
Number of papers with 2 questions marked = 0. Tests numbers = 
Number of papers with 3 questions marked = 18. Tests numbers = 2-16, 18-20
18 of 18 complete
*******************************
** No tasks currently "out" ***

---- 8< ----

Marks written to "marks.csv"
$ A=`cat marks.csv  | wc -l`
$ bash -c "[[ $A == 20 ]]"
ERROR: Job failed: exit code 1

That is, not 20 lines in marks.csv. And note 18 of 18 complete but it should be 19. Here's the usual good run:

********************
** Completion data **
Produced papers: 20
Scanned papers: 19 (currently)
Completed papers: 2-20
Identified papers: 2-20
Totalled papers: 2-20
Number of papers with 0 questions marked = 0. Tests numbers = 
Number of papers with 1 questions marked = 0. Tests numbers = 
Number of papers with 2 questions marked = 0. Tests numbers = 
Number of papers with 3 questions marked = 19. Tests numbers = 2-20
19 of 19 complete
*******************************
** No tasks currently "out" ***

----- 8< -----

Marks written to "marks.csv"
$ A=`cat marks.csv  | wc -l`
$ bash -c "[[ $A == 20 ]]"
$ file -b marks.csv  | grep ASCII
ASCII text, with CRLF line terminators
$ plom-finish reassemble -w 1234