The problem with Android is that other applications can display windows on top of your applications, thus making it appear like they came from the current application (Textra in this case).

Although not our issue, we are happy to help you locate the offending application (if only to prove this to you and gain your trust back).

Send us a screenshot of Android Settings > Apps > (cog) > Draw over others app (sometimes called 'Apps that can appear on top') you'll see a list of applications that use this Special Permission (that allows a window to be shown on top of Textra SMS, thus tricking you into thinking it's Textra SMS).

OR it's your keyboard app.