I have a master item that gets item data like this (HTTP agent):

{"strings":["AutoFullBackup-20190912_160006","AutoFullBackup-20190913_160125","AutoFullBackup-20190914_160232","AutoFullBackup-20190915_160343","AutoFullBackup-20190916_160453","AutoFullBackup-20190917_160606","AutoFullBackup-20190918_160716","AutoFullBackup-20190919_160835","AutoFullBackup-20190920_161000","AutoFullBackup-20190921_161111","AutoFullBackup-20190922_161219","AutoFullBackup-20190924_140600","AutoFullBackup-20190925_140714","AutoFullBackup-20190926_140825","AutoFullBackup-20190927_140940","AutoFullBackup-20190928_141050","AutoFullBackup-20190929_141200","AutoFullBackup-20190930_141312","AutoFullBackup-20191001_141426","AutoFullBackup-20191002_141541","AutoFullBackup-20191003_141651"]}

Then I have 2 dependent items which turn to unsupported with an error:

• Item: Number of backups
  Preprocessing: JSONPath: $.strings.length()
  Error:

  Preprocessing failed for: {"strings":["AutoFullBackup-20190912_160006","AutoFullBackup-20190913_160125","AutoFullBackup-201...
    1. Failed: cannot extract value from json by path "$.strings.length()": object not found starting with json path: "length()"

• Item: Last backup
  Preprocessing: JSONPath: $.strings[-1]
  Error:

  Preprocessing failed for: {"strings":["AutoFullBackup-20190912_160006","AutoFullBackup-20190913_160125","AutoFullBackup-201...
    1. Failed: cannot extract value from json by path "$.strings[-1]": unsupported character in json path starting with: "[-1]"{color}

However, when I copy-paste the data from the master item into the Preprocessing test dialog of the web UI.. I do get the expected results:
For Item 1 I get "21" as test result
and for item 2 I get "AutoFullBackup-20191003_141651" as test result

Steps to reproduce:

1. Create master item, and feed it with this JSON string (I get this data using a HTTP agent, but I assume a trapper would suffice):

   {"strings":["AutoFullBackup-20190912_160006","AutoFullBackup-20190913_160125","AutoFullBackup-20190914_160232","AutoFullBackup-20190915_160343","AutoFullBackup-20190916_160453","AutoFullBackup-20190917_160606","AutoFullBackup-20190918_160716","AutoFullBackup-20190919_160835","AutoFullBackup-20190920_161000","AutoFullBackup-20190921_161111","AutoFullBackup-20190922_161219","AutoFullBackup-20190924_140600","AutoFullBackup-20190925_140714","AutoFullBackup-20190926_140825","AutoFullBackup-20190927_140940","AutoFullBackup-20190928_141050","AutoFullBackup-20190929_141200","AutoFullBackup-20190930_141312","AutoFullBackup-20191001_141426","AutoFullBackup-20191002_141541","AutoFullBackup-20191003_141651"]}
   

2. Create dependent item:

• Name: Database auto-backup: number of available backups
• Type: Dependent item
• Master Item: (key of master item)
• Type of information: Numeric (unsigned)
• Preprocessing:
• JSONpath: $.strings.length()
• Discard unchanged with heartbeat: 1h (don't know if this is relevant)

1. Use the Preprocessing Test-link next to the JSONpath rule:

• Set the above JSON string as value and press test
• See that the result is 21

1. Create another dependent item:

• Name: Database auto-backup: time of latest backup
• Type: Dependent item
• Master Item: (key of master item)
• Type of information: Numeric (unsigned)
• Units: unixtime
• Preprocessing: (not sure if anything after the JSONpath is relevant)
• JSONpath: $.strings[-1]
• Regular Expression: {{^AutoFullBackup-([0-9] {4}

  )([0-9]

  {2})([0-9]{2}

  )_([0-9]

  {2})([0-9]{2}

  )([0-9]

  {2}

  )$}} - \1-\2-\3 \4:\5:\6

• Javascript: return new Date(value).getTime()
• Custom multiplier: 0.001
• Discard changes with heartbeat: 1d

1. Use the Preprocessing Test-link next to the JSONpath rule:

• Set the above JSON string as value and press test
• See that the result equals AutoFullBackup-20191003_141651
• You can try the "Test all steps"-link. This should result in a valid unixtime.

Result:
For item 1: Turns unsupported with error:
Preprocessing failed for: {"strings":["AutoFullBackup-20190912_160006","AutoFullBackup-20190913_160125","AutoFullBackup-201...
1. Failed: cannot extract value from json by path "$.strings.length()": object not found starting with json path: "length()"

For item 2: Turns unsupported with error:
Preprocessing failed for: {"strings":["AutoFullBackup-20190912_160006","AutoFullBackup-20190913_160125","AutoFullBackup-201...
1. Failed: cannot extract value from json by path "$.strings[-1]": unsupported character in json path starting with: "[-1]"

Expected:
Item 1 value: 21
Item 2 value: 1570112211