Greetings from Syncfusion.
We have checked your code snippet and background color will not apply for the helper text with the provided code snippet. We have tested using the latest version (18.104.22.168) and attached the screenshot below. We suspect that, you may set the background color to the other layout.
Please confirm us whether you using only the provided code snippet in your application? Still if you are facing any problem please revert us by modifying the provided sample which will be helpful for us to assist you earlier.
If you want to set color to the background you can set using ContainerBackgroundColor as in the below documentation link.