Can't change weight for custom field multiple choice options when all originally created with same weight

I created a custom data field as a multiple choice checkbox list. I set the weight for all of the options in the list as "1" thinking that this would cause the list to be in alphabetical order, but it didn't work. I tried to change the weights of individual options, but when I change the weight of an option and save it, the weight reverts back to "1."

I tested another list that had not been created with all options' weight set to "1" and changes worked as expected.

I created a new multiple choice list with all options' weights set to "1" and again found that I could not later change the weights of options.

I then created another multiple choice with all options' weights set to "2" and found that I could not then change the weights of options.

Finally, I created a multiple choice list with some options' weights set to the same values and others set to weights that only they had. This time I could successfully change the weights of any of the options.

So it seems like this only happens when the multiple choice list is originally created with all weights equal.