Hi OSTexo, the problem is that the edges in the RST hole are not actually free unattached edges, there is a micro thin surface joined to them.
So for instance if you select this face:
And then delete it, you will see this little left-over piece:
That looks like a curve, but if you select it you'll see that they are actually surfaces. Since those are joined edges that's why they can't be untrimmed.
To fix it, drill in to select just the big face, use Edit/Separate to break it out from the others, then use Edit/Hide to hide it, now you can select and delete the little fragment piece, then show the big piece again, select everything and join it back together, and now you can untrim it like you want.
- Michael
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