Hi Ali,
We afraid.We couldn't able to reproduce the issue as you mentioned. We tried to place the AutoComplete textbox inside the UserControl. Unlike you mentioned, it doesn't reproduce any issue. Attached is the sample in which we tried to reproduce the issue.
AutoComplete Demo SampleIf anything we made wrong in reproducing the issue please correct us. In the meantime, please download our sample and do let us know if you are able to reproduce the issue in our sample too.
Let us know if it helps.
Thanks for using Syncfusion products.
Regards,
Gokulkumar B