In short, the white rook from D1 can take the black rook at E1, so this is a wrongly detected mate.

Here is the move history.

1. Pd2-d4. Nb8-c6
2. Pd4-d5. Nc6-e5
3. Bc1-f4. Ne5-g4
4. Pe2-e4. Pe7-e5
5. Qd1xg4. Pe5xf4
6. Qg4xf4. Ng8-f6
7. Nb1-c3. Pa7-a6
8. Ng1-f3. Ph7-h6
9. Bf1-c4. Pb7-b5
10. Bc4-b3. Bc8-b7
11. Pa2-a4. Pb5-b4
12. Nc3-a2. Pa6-a5
13. Pc2-c3. Pc7-c5
14. Pd5xc6. Pd7xc6
15. Pc3xb4. Pa5xb4
16. Ke1-g1. Pc6-c5
17. Na2-c1. Bf8-d6
18. Nc1-d3. Bd6xf4
19. Nd3xf4. Ke8-g8
20. Nf4-g6. Rf8-e8
21. Rf1-e1. Bb7xe4
22. Nf3-e5. Be4-d5
23. Ra1-d1. Pf7xg6
24. Ne5xg6. Re8xe1+

Hello,

Thank you very much for signaling. The bug has been confirmed and will be fixed in a next update.