Out of curiosity, why is "tutan lagon" in the accusative? (That is, why does it end in "-n"?)
I thought the accusative was needed to show a motion/transfer towards something, especially when the usage without the accusative has a distinctly different meaning.
What would it mean without the accusative? That is, what would "Ŝi naĝis trans la tuta lago." (without the "-n") mean? Doesn't it mean the same thing? If so, then why use the accusative here at all?
I don't have an opinion.
My sense is that this is a software bug and would need to be reported as a bug report not as a "flag" or "report a problem" - and to do that you have to navigate to support dot duolingo dot com -- and report it. Sounds like a pain in the rear -- and not worth doing multiple times.