Inconsistent canonical tautomer on repeated application

[darintay]

Describe the bug
Tautomer canonicalization seems to be occasionally inconsistent, repeated canonicalizations will produce different results.

To Reproduce

Repeatedly pick the canonical tautomer for Serine.

from rdkit import Chem
from rdkit.Chem.MolStandardize import rdMolStandardize

enumerator = rdMolStandardize.TautomerEnumerator()

mol1 = Chem.MolFromSmiles('C([C@@H](C(=O)O)N)O')
mol2 = enumerator.Canonicalize(mol1)
mol3 = enumerator.Canonicalize(mol2)

print('\n'.join([
    Chem.MolToSmiles(mol1),
    Chem.MolToSmiles(mol2),
    Chem.MolToSmiles(mol3)
]))

Output:

N[C@@H](CO)C(=O)O
NC(CO)C(=O)O
NC(C=O)C(O)O

Expected behavior

I would expect mol2 to be the same as mol3. The canonical tautomer of the canonical tautomer should still be the original canonical tautomer (unless we hit MaxTautomers or something like that, which is not occurring here).

Instead mol2 and mol3 differ in where they've put the double bond.

Configuration (please complete the following information):

• RDKit version: 2020.09.3
• OS: Ubuntu 20.04
• Python version (if relevant): 3.7.9
• Are you using conda? Yes
• If you are using conda, which channel did you install the rdkit from? conda-forge

[ptosco]

@darintay This does look like a bug to me, I'll take a look. Thanks for reporting it!