You may notice that there's a small portion of white space underneath the 'value' button. This is because if I expand it to take up the entire space of the flex container, the text will jump to the top. As shown here, nothing has changed except for the relative size of the button:
There does seem to be a bug here, but it stems from a poor approach to configuring the instanced view: instead of setting an extremely high basis, set the basis back to 32px and change the grow property to 1. I'll open an internal ticket to see if we need to do anything about the basis scenario.
I don't think this applies just to your situation. I'm in 8.1.22 and added a button into a flex container and am seeing the same thing. EDIT: This appears to only be an issue in the designer; in Chrome it shows up correctly in the centre
(it's more obvious obviously the taller the button)
Thanks for all the replies everyone.
Zach, I'm sorry but I'm not going to open that zip file. Perhaps you could describe what you did.
cmallonee, I changed these properties as suggested, it had no effect.
nminchin, I'm marking you as the solution. As long as it looks good when the project is actually in a web page, then I don't really care how it looks in the designer. I didn't even think to actually test it in the right environment and instead spent all my time trying to configure the settings to get it to look right. Lesson learned, test in both environments.
With that said, I still think it's an issue. It just doesn't really matter anymore.