Hi there!
I tested this and actually found an Oops error to be occurring in my test environment when attempting to add the OneTime product to the quote.
As such, I have opened case CORE-14168 with our Development Team to have this reviewed for future releases.
Whilst I cannot provide an estimated time for completion for this, once we resolve cases and push features they are available at our change log, here:
http://changelog.whmcs.com/
I apologize for the inconvenience, and appreciate your patience as we work to resolve this.
Thanks again for taking the time to report your findings.