Sentry includes a number of built-in data scrubbers. When enabled, these look for things such as "password" or credit card numbers. If you have a field which is being scrubbed (and you're certain that it won't contain sensitive information) you can add it as a safe field which will not be scrubbed.
If your data is not sent in a structured format, Sentry will not be able to detect the field name. For example, if you're sending a JSON payload as a raw string, scrubbing cannot be disabled on it. To resolve this you should ensure you're sending rich structure data whenever possible.