form replaces list slot with length 1 with the item in the list

[amn41]

User raised this on the forum and I was able to reproduce using their example code.

You can see this happen if you run with rasa shell --debug

2021-04-05 16:15:28 DEBUG    rasa.core.processor  - Predicted next action 'order_pizza_form' with confidence 1.00.
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Activated the form 'order_pizza_form'.
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Validating pre-filled required slots: {'toppings': ['pepperoni']}
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Validating extracted slots: {'toppings': ['pepperoni']}
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Validating user input 'UserUttered(text: I want a pepperoni pizza, intent: pizza, entities: pepperoni (Type: toppings, Role: None, Group: None), use_text_for_featurization: False)'.
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Extracted 'pepperoni' for extra slot 'toppings'.
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Validating extracted slots: {'toppings': 'pepperoni'}
2021-04-05 16:15:28 DEBUG    rasa.core.actions.forms  - Deactivating the form 'order_pizza_form'

Rasa Version     : 2.3.1
Rasa SDK Version : 2.3.1
Rasa X Version   : 0.37.0
Python Version   : 3.7.6
Operating System : Darwin-19.6.0-x86_64-i386-64bit