The reason it hasn't is that water generally doesn't flow uphill.
I'm too lazy to calculate the approximate volume of fresh water that represents, based on the lake's surface area. Pretty safe to say that the rest of the lake's volume, at and below sea level wouldn't be rendered brackish too easily.
Although, given China's penchant for environmental accidents, I wouldn't put it past them, if all that additional lake water above sea level were to get skimmed off the top by an accidental spill into the ocean.
The danger is ballast water held by ships, but like Panama they'll likely forbid all water discharge while in the canal.
Generally, the big risk with elevated freshwater lakes connected to the sea by locks isn't saltwater contamination, but the disastrous flood that would happen if all the lock gates were opened at the same time. A lot of work was put into the safety measures, most of which have been removed due to cost in recent decades: https://en.wikipedia.org/wiki/Panama_Canal_locks#Safety_feat...
If a flight of locks was completely opened, Gatún would promptly drain out of them, wiping out everything downstream. You could imagine the entire canal would be offline for months to years as infrastructure was rebuilt and the lake was refilled by rainwater.