If you fix the activity bug I think that I'll fall in love with you!
Waouhhh... So I'm gonna fix it immediately -


But, I don't encounter any bug in saving polylines now...
I have tried to create a new polyline with an activity: I save the polyline then I save the typ file. And the polyline is still in the file. I close TYPViewer, reopen it with the saved file and the polyline with activity is still here.
Then I have made the same test but with an existing polyline: I modify a polyline without activity in a polyline with activity: all works correctly like in previous test.

Are you sure that the line with activity disappeared?
In TYPViewer , polylines are sorted by activity: first , all the lines without activity ( sorted by types) then all the lines with "Walking" activity, then all the lines with "Hiking" activity etc..
So, when you change activity for a line, this is moved to the other lines with the same activity.
Please tell me exactly what change you have made (creating a polyline or modifying one existing polyline) , in which typ file