I wrote it this way in my manuscript:
We construct a 25-order planar graph with minimum degree 4.
My grammar checker (languagetool.jar) is reminding me to use "an" instead of "a." I'm a bit unsure, so I'm asking here.

I suspect the reason could be that the grammar checker doesn't recognize "25" and instead directly recognizes "order."
I'm not sure if the issue is with this software or if it's related to using "number-noun" constructions (as there might be specific rules regarding this).