Not sure what you're saying. Keyword
:prefix does not change the names of variables that are in the group. It does not affect the variable names. All it does is declare that the group uses that prefix.
And if option
custom-unlispify-remove-prefixes is non-
nil then variable names are shown in Customize buffers without their prefix.
Is that what you are trying to describe: that you expect to see the names without prefixes but you see the prefixes (in Customize buffers)? If so, is that option non-
If a Customize buffer already exists showing a variable then it does not get updated to reflect a change in that option value. But if you kill that Customize buffer and re-create it then you should see the option value being respected.
If this does not answer your question, please try to clarify what you mean: what you see and what you expected to see instead. Consider providing a recipe.
See the Elisp manual, node Group Definitions for the description of