MTextobject in modelspace, ok:
Is it visible in Layout, but not in PDF/Paper?
Same problem with all Objects on this Layer? => Check the PLOT property of this layer (not a Hint for you, just for other readers in the future)
All Objects not visible in this viewport? Check the new (LOL) VP FREEZE property of this viewport (not a Hint for you, just for other readers in the future)
ONLY (this) MText object?
Is it near to the viewport boundary located?
One eye to the STYLE, Font and special characters!
I saw that in the past. The Text or MText was near a corner of the viewport.
You could see it in Layout, but the content switched or disappear by plotting (wow).
I don't remember about the issue exactly, but it was a Font and perhaps Content problem.
Tell me if i should start my research.