Not quite sure how to fix that but I guess in the meantime you should get into the habit of hitting backspace before entering your value in. Not exactly what you wanted to hear but it will definitely work.
Have you checked your units being used? You might want to specify your units instead of just entering a number.
Very bad behavior by the edge flange tool. It's always been that way. If you enter the flange length in advance, you still have to select a direction in the graphics area and that click gives the flange a length, which overrides the value that you entered. The workaround is to remember to select the direction first and enter the length second.
I'm thinking you clicked in the beginning of the size box while the flange was still rubberbanding on the screen and then typed your value.
If you press Enter after doing this, the entire entry is highlighted.
Unfortunately, the current size isn't highlighted [when you start the command], so by clicking in the entry box, your cursor is placed where you clicked and there are not changes made to the text already there.
As a suggestion, press Ctrl+A after you click into the size box. This will highlight all of the existing and everything will then be deleted when you enter your required size.