Even if the faceplate has enough room to tighten on a 25.2 bar, it isn't compatible. The curvature is different so the clamping force won't be evenly distributed, and will distort the clamp plate or handlebar causing stress above what either is designed for.
An analogy can be drawn to an egg shell. If you put the egg in your hand and squeeze evenly it'll be hard to crack, but if you squeeze just on two sides it'll be fragile as, well, an egg.
If you can't bring yourself to replace the bar or stem, try to locate some material exactly 0.4mm thick to use as shim stock to make up the difference and match the bar to the stem.
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