I just notice this oddity too on the updated version of AE currently mine is at 15.0.0 (Build 180)
In short when you grab the “TIME NAVIGATOR END” and try to zoom in on the sequence it suddenly goes from second to frames….
If anyone figures how to just keep it seconds it would be great!
Here is the sequence with seconds

Here is the same sequence when it pops to frames

Any ideas folks??
-David