Option values with underscores and numeric beginnings for custom select fields display multi-valued values when viewed, but work fine when edited, but only when one option value is without the underscore

Might get a vote for most obscure bug ever...

Ok, how to describe this...

Create a custom field for a specific activity type. In my case it was a custom activity type called TestActivity. Make the field an alphanumeric select field.
Define some option values for it that are numbers except for an underscore, but make the first one NOT have an underscore, e.g.

label: option 1
value: 1

label: option 2
value: 1_2

label: option 3
value: 1_3

Create an activity of the type in question for some contact. Choose option 2 as the value.
Go back to view this activity. It will display "option 1, option 2" as the value for the field. The expected display is, of course, "option 2".