I think self-contradictory might be a good description.
an act or instance of contradicting oneself or itself.
a statement containing contradictory elements.
You wanted something with a neutral connotation. I think, in this case, it can be neutral based on your phrasing. Something like
Your argument seems to be self-contradictory to me, because in this
sentence you say X which seems to imply you are against Z, but in this
other sentence you say Y which seems to imply you are in favor of Z.
If you simply state that the person's argument is self-contradictory that could be considered negative because it implies that they don't know how to construct a logical argument. Or, at least, in the crowd I run in, it would be considered negative. But if you phrase it with "it seems to me" or "I don't understand because X" you are opening it up to your own misinterpretation and asking for clarification.
Another word you might consider is ambiguous.
open to or having several possible meanings or interpretations; equivocal: an ambiguous answer.
Linguistics. (of an expression) exhibiting constructional homonymity; having two or more structural descriptions, as the
sequence Flying planes can be dangerous.
of doubtful or uncertain nature; difficult to comprehend, distinguish, or classify:
a rock of ambiguous character.
lacking clearness or definiteness; obscure; indistinct: an ambiguous shape; an ambiguous future.
If you are trying to be neutral, again, phrasing would help. I think "ambiguous" is much less "negative by default"--more of a declaration that the person's argument is not completely clear rather than faultily constructed. But I would still say something like
This argument seems ambiguous to me, because in this sentence you say X which seems to imply you are
against Z, but in this other sentence you say Y which seems to imply
you are in favor of Z.