Thanks for contacting Syncfusion support.
We are unable to reproduce the reported issue with UploadBox OnComplete and OnRemoveFile server side event. We have attached a sample prepared in build version 12.4451.0.24, please ensure whether the sample works fine at your end. Also can you share us defect code or describe the scenario to reproduce the issue or modify the shared sample to reproduce issue at our end. So that we can provide you a better solution.
You can download the sample from following link:
Kindly let us know if you have further queries.
Thanks for the update.
We have created a new incident for your reported query. Kindly follow up that incident for further assistance.
Please let me know if any concerns.