When selecting text and copying it on my machine which is then synced to my phone, I get a "Copied to clipboard" toast (not sure if that's created by Android when setting text on the clipboard or KDE Connect). Starting plasmashell (more precisely: the clipboard/klipper applet) sets clipboard text (restores it from history?) causing said toast to appear on my phone whenever I start plasmashell. The question is: 1.) should klipper not set clipboard text if it didn't change? but then X is probably weird and we cannot be sure and *have* to do that 2.) should KDE Connect not set clipboard text if it didn't change? but then that might also cause trouble elsewhere? if we can even *read* the clipboard in the first place...
Klipper cannot do anything about that.
Which version of Android are you using? KDE Connect doesn't explicitly show anything on Android, so maybe it's something new they added in Android O or even some other app you have installed.
Seems to be BlackBerry's keyboard app doing that.