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