Well it’s seems to be a bit tricky. At the moment i cant reproduce the bug again. I changed some lines of code that shouldn’t be related to blynk but now its working fine. this evening i’m going to see if i can reproduce it again, and see what made the difference.
and i’m sorry for the inconvenience 
I have the same problem.
First set label from Arduino side: Blynk.setProperty(V39, “onLabel”, “oldlabel”);
In second time changed label in: Blynk.setProperty(V39, “onLabel”, “newlabel”); sometime take “newlabel”, but when i stop an restart project on app, the Button label return in “oldlabel”.
I change manually Label on “Button Settings” on the App, but when project start come back “oldLabel”.
App on android, version 2.15.3
P.S. Not all Buttons on the project (about 30) give this problem
I use Blynk server
it seem that Blynk server send to App new label but somewhere on the App remain “oldLabel” and come back on Button.
P.S. Reinstalling App don’t solve problem.
Solved in this mode:
Delete from App, widget Button on V39, send form Arduino side the “newLabel” of V39 to Blynk server, add new Button on V39.
Seems that App send to Blynk server “oldlabel” and overwrite “newLabel” but if this Button don’t exist this newLabel still stored right.
I find where is the problem:
When i change property button (Blynk.setProperty(V1, “offLabel”, “newlabel”); on the app is updated but in the server still oldvalue. I saw this on my custom server (opening file email@myemail.com.Blynk.user.
When the android app restart, it upload from Blynk server the old value
Server ver. 0.28.4-java8
App ver. 2.16.3