We have noticed this too. Super frustrating. You should be able to write the full variable value to the log, but that is frustrating, annoying and a bad solution. So while it may help out in the meantime, I really hope that the Mendix development team corrects this somehow.
Maybe a workaround: Have you tried running your app from Eclipse? See you get the full value there.
I fully agree! I have wondered about this myself as well. I guess it is performance related?