Hello there, @mikeg ...
I am not completely sure I would consider this a bug within Canvas. In doing some searching in the Canvas Guides, I found this documentation:
How do I grade one quiz question at a time in SpeedGrader?
You'll notice that underneath the heading "Assign Grade" (and the accompanying image under the heading), the following sentence reads (highlighting is my own):
"To assign a grade for a question or to revise the grade for an automatically graded question, enter the score in the points field [1], then click the Update Scores button [2]."
So, based on this information, this process would be "by design".
If you want to see this behavior changed within Canvas, then you'd want to consider submitting a Feature Idea here in the Community so that other members here could also evaluate it and determine if this is something they would also like to see changed in Canvas. Here are some documents for you to look through:
Hopefully this helps to clarify things. Please let Community members know if you have any other questions about this...thanks.
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