In the activity log, it can be seen that a lead has been added to a custom object, but in the custom object tab there is no data.
The reason why that it appears as the custom object data is missing for a lead is because the custom object data becomes disassociated to the lead and associated to another lead when the custom object data is pushed into Marketo.
When the custom object data is pushed to Marketo intended for lead A with the same dedupe field value of an existing custom object record associated to lead B, it disassociates it from the lead B and associates it with lead A. Therefore, when looking at lead B that has had an 'Add to Custom Object' activity, but does not have the Custom Object data visible in the Custom Objects tab, it is because the custom object data was associated to another lead.
Since the custom object data is moved to this lead, it is no longer visible in the custom object tab for Lead A.