Boolean checkbox fields in Salesforce (true/false).
Outreach custom fields.
SFDC forces a strict lowercase value for their boolean checkbox fields. They force the true and false value only, and is case-sensitive when syncing:
true = checked
false = unchecked
If you're syncing and mapping any Outreach custom fields to a Salesforce boolean field, the values in Outreach must be lowercase true or false.
We also recommend creating a validation field in Outreach for those specific fields where the only pick-list values are true or false.