VSCode Vim Search and Replace capture group

Edit: I just tried this in the VSVim extension in VS2019, and it worked as expected. I'm starting to think that VSCodeVim extension for VSCode doesn't handle captures properly?

I am trying to search my typescript file for a list of variables that have not yet been assigned an initial value, and set it to = null.

  private __requestor: Req;
  private __feedback: FeedbackObject;
  private __dueDate: Calendar = null;
  private __priority: number = NaN;
  private __bigTest = TestObject;

I am using the following command: :%s/(: [a-zA-Z]+);/\1 = null;/g

I expect the output to stick = null on lines 1, 2 and 5, but instead it sticks a \1.

Expected:

  private __requestor: Req = null;
  private __feedback: FeedbackObject = null;
  private __dueDate: Calendar = null;
  private __priority: number = NaN;
  private __bigTest: TestObject = null;

Actual:

  private __requestor\1 = null;
  private __feedback\1 = null;
  private __dueDate: Calendar = null;
  private __priority: number = NaN;
  private __bigTest\1 = null;

Is there something wrong with my regex/search & replace command? It looks similar to the other S&R commands that use capture groups in examples I've seen, and I haven't seen any settings for "enabling" capture groups.