Numerical question with optional units: multipliers not used if numerical value is correct for Unit 1

In numerical question with "Units are optional" setting, if the submitted answer contains ANY unit, and the correct numerical answer for Unit 1 (multiplier 1) is given, the answer is graded as correct. This seems to be unwanted behavior, because if the correct answer is "1 m", then "1 dm" should not be correct.
Even the instruction for unit handling says: "If the unit IS ENTERED, it is used to convert the response to Unit 1 BEFORE grading."
Of course, the answer "1" is correct, which is expected.

Steps to reproduce:
1. Create numerical question, with "Units are optional" setting.
2. Create a correct answer with value 1.
3. Create the following units:
Unit 1: m Multiplier: 1
Unit 2: dm Multiplier: 10
Unit 3: cm Multiplier: 100
4. Preview the question.
5. Enter "1 dm" or "1 cm". The grading will report it as a correct answer.