While DT Register supports partial payments, it does NOT send any payments to ANY gateway labeled as such. If your event is $370 and you allowed partial payments, the amount being paid is what is sent to the payment gateway. We do NOT send the gateway any further amount due and do NOT send anything that would cause the payment to be classified as pending.
My assumption is that the cards used in these couple of instances were not approved for a full amount. Maybe some type of gift cards??
You would need to follow this up with the user and Authorize.net in more detail. Whatever amount DT Register calculates as being due is the only amount that is ever sent to Authorize.net... and if that amount is not paid, the record is not created.
As Authorize.net has the amount as "Held", that tells me that it couldn't approve the full amount. You need to modify your Authorize.net account settings to disable that type of functionality. Set it up so payment is either accepted or denied. No inbetween. That's where changes are needed. Thanks!