Sorting function randomly duplicating values on columns with missing values

Minimal example with the following 2x2 table in CSV, where the first row misses its second value :

2
1,X

Selecting everything and issuing :sort "+#A" (for sorting according to numeric values in first column), I sometimes get the expected results

1
2,X

but sometimes the X value in the second column gets duplicated

1,X
2,X

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