I've been having a problem with this for quite some time on the Android app. I use the Samsung Keyboard as my default. To reproduce the issue basically every time, I can reply to a message that already has my HTML signature (that was created using the built-in editor) embedded, and as I am typing my reply message, at some point it appears to just switch to non-rich text mode or something, because all of the formatting of my signature is gone, the text goes to double spaced on the line breaks, and the picture in the signature is replaced with the square "obj" box. Hopefully this information is helpful. I would sure like a resolution to this.