The default altitude when I plan a new mission, is always 98 ft. ALWAYS save your missions for this very reason. You can look back and see what errors you made. In addition, you might want to fly the same mission again.
Here's what appears to be what happened in your case. You laid out your waypoints (which defaulted to 98 ft). You then set your first waypoint to 398 ft and didn't look at/change the succeeding waypoints to an altitude different from the default 98 ft, assuming they would ALL be 398 ft. This makes me wonder how much time you spent reading the manual or watching videos on how to use Litchi. Not trying to be critical but reading the manual and having a thorough understanding of the Litchi app is of paramount importance to using it successfully. Attention to detail is CRITICAL. You can't "assume" anything. You saying it descended without you telling it to is incorrect. By not checking the altitude at each waypoint clearly displayed on your screen, you DID, in effect, tell it to descend, albeit unintentionally.
Read the manual from front to back until you understand it (It's short (43 pages)) and is downloadable here:
For IOS:
Litchi for iOS February 2017.pdf
For Android:
Litchi P3 1_10_16.pdf
It would also do you well to watch this video: