Specifying a valueMap on a field isn't sufficient to enforce that submitted values fall within the valueMap - we allow fields to use valueMaps as essentially a suggested value, but also allow additional values to be applied (for example via a ComboBoxItem with addUnknownValues set to true).
From the docs for DataSourceField.valueMap:
No comment on the second warning. If you can get it to reproduce we'd recommend looking at the RPC Tab in the dev console to see what the request / response looks like, and the server logs for the transaction. Hopefully this'll reveal more information about what's going on...
From the docs for DataSourceField.valueMap:
...To enforce that a field should be constrained to only the values in the valueMap, either declare field.type as "enum", or use a ValidatorType of "isOneOf" with explicitly listed values. Otherwise, although a normal SelectItem control will only allow values from the valueMap to be entered, other controls such as a ComboBox will allow other values to be entered.
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