I think I found out what the problem was. If the length of the Description field in the database exceeds certain amount (I think more than around 10000), the indexer will not work correctly. To test this, I changed the input query in DB Connect to only retrieve the first 500 characters of the Description field from the database (on the same set of data), and everything works fine. The length of the Description field of erroneous indexed events was way more than 10000, for example one had a length of around 17000 characters (the correctly indexed event with maximum length had a length of 10136).
I think this is a bug and relates to the maximum size of the variable type of the fields.
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