I get a weird behavior formating a textbox in a group header. If the textbox is set to default shows the data, but so soon how set the format to percentage (or anything else) the complete row (group header) won't show.
Manually changed the height of the row and it's working. I don't know if was problem with the screen high resolution (2560 x 1440), I saw similar issues with some windows in SSMS.
Use Expression instead of format.