Redefine style changes Grep Style
When redefining a style based on changes to an existing style (and that existing style has a GREP style in use), the style parameters of the style or grep style are changed. Whereas, if you edit a style directly and apply the same changes, the grep style is unaffected. See video.
In the 1st case in your video, you have applied a local override to the Style by changing. That's why you see a + in the name of the Style. When you Redefine that Style, the overrides go away and the Style gets redefined with the new attributes.
However, in the 2nd case, you are Editing the Style directly by opening the Style dialog. So any changes you make in the dialog automatically becomes part of the Style itself and there is no need to Redefine it from the panel menu.
It is working correctly.
Adobe InDesign team
Keith Cross commented
I replied to the email that was sent, that reply isn't showing here yet...but I noticed something else...
Another style using the same two font styles (medium and medium italic) is using a different grep style condition (look ahead instead of look behind) and works without changing the weight of the style when the style is redefined.
The style causing the error uses this to use a character style the uses the Medium Italic: (?<=:).+\r
The style which is NOT causing an error uses this to apply a character style that uses Medium (meaning the paragraph style is set to Medium Italic): s: ^.+(?=:):
Is there a problem with using one vs. the other?