I really should download 2017.3 to see if I can repro this, cause I've had no luck doing this in 5.3.4 or 2017.1.
The only real things I can think of that will solve this are...
In SuperTextMesh.cs, if you ctrl+F "OnFontTextureRebuilt", you'll find a function that should be getting added to an event called Font.textureRebuilt... this event should be getting added under Init(), and removed under UnInit()... Maybe the problem is that it's getting added too many times, and if the textbox is being enabled/disabled, the event handlers are getting confused?
Send me an email through my website, and I'll send you a beta build! I'll try making it so these handlers are only called once no matter what.