You either haven't made sure the memory you've allocated for the sprites has been cleared to 0xff000000 or you're not plotting black into the background or the font didn't load the same way in release mode, or something else. But it's impossible to tell without the exact code that you have that does this.
I've mentioned before that VC runtime in debug mode does things to clear memory to magic values so you see mistakes sooner (it's allowed to do this and it really helps). Looks like your code is probably dependent on these magic numbers in some way so you are seeing the problems in release mode instead. It's usually the other way round, but hey!
Have you tried using the debugger? You can still use it a bit in Release mode. F9 sets a breakpoint, then when you run it the code will stop at that line and you can inspect all the variables and memory.
Jim