I drew each of three related tautomers in MarvinSketch (using the online version, which I believe is 5.3.8) and then selected Tools -> Isomers -> Tautomers -> All Tautomers.
The three compounds are listed here as SMILES:
Compound A: O=C1CC(=NO1)C1=CC=CC=C1
Compound B: O=C1ONC(=C1)C1=CC=CC=C1
Compound C: OC1=CC(=NO1)C1=CC=CC=C1
When I input Compound A, the only tautomer found is Compound A.
When I input B, the only tautomer found is compound B.
When I input C, tautomers B & C are found.
Shouldn't all three tautomers be returned, regardless of which compound is used as the input?