There is something going on within your environment. Let's start with basics. Just FWIW, this code hasn't changed since version 1.5 of SF. So I really don't think that SF is doing anything here, otherwise the entire SF user base would be reporting this same issue.
Your localization environment of your application could have an effect, but I am going to need more details to help you work through this. If you look at the SF source code, the privates attempt to retrieve the localization value.
For example:
Private _NewButtonText As String = Localization.RetrieveTextValue("UI_New", "New")
This is done at this point to allow the value to be overwritten via the designer. So it sounds to me like you have one of several issues going on. Before I get into each point, do the following:
1. Create a new SF application
2. Create a form and drop on a Maintenance Form Toolstrip
3. Build the project
4. Run the project
5. Get out of VS
6. Re-open the dialog with the MFTS
7. At this point, are the values showing the UI_New, etc. keys?
if the above did not fail, then you have something going on within your project, if not, then you have something going on within your development environment. This will help to know which way to nudge you.