Assuming that the percentages you express in your question reflect AncestryDNA tests by you, your mother and your paternal grandmother, then I think it is possible that your paternal grandfather was close to 100% Chinese, and that close to 50% of his Chinese DNA passed to your father, and that this manifested as close to 25% in you.
However, I strongly recommend seeking paper records of his birth before accepting this hypothesis.
Since you know nothing about your paternal grandfather it remains possible that he was 100% (or close to 100%) Chinese.
If you were to find a birth record for him that suggested otherwise then you would need to try and reconcile that against the DNA evidence.