Short answer. No. Article 1105 of the Civil Code provides that a partition including a person believed to be an heir, but who is not, is void only with respect to that person. The shares actually belonging to the real heirs remain valid; only the non-heir's inclusion needs to be corrected.
What the law says
A partition which includes a person believed to be an heir, but who is not, shall be void only with respect to such person.
Civil Code, Article 1105 — A Partition Naming a Non-Heir Is Void Only as to That Person. Read the full provision →
The defect is contained, not contagious
Article 1105 states the rule plainly: a partition which includes a person believed to be an heir, but who is not, shall be void only with respect to such person. So discovering that one participant in the division was never actually an heir does not automatically unravel the whole partition. The law isolates the defect to the mistaken inclusion itself, leaving the rest of the division, the shares correctly allocated among the genuine heirs, standing on their own. The partition as a whole is not treated as fatally flawed simply because one participant should never have been part of it.
What happens to the share the non-heir received
Because the partition is void as to the wrongly included person, whatever share that person received under the partition does not validly belong to them. That portion should properly be treated as still forming part of the estate to be distributed among the actual heirs, since the non-heir never had a real right to inherit in the first place. Sorting out what happens to property already transferred or spent by the mistaken participant is a separate, practical problem from the legal question of the partition's validity, and often the more difficult one to resolve.
Why the law protects the partition of genuine heirs
Treating the whole partition as void whenever one participant's status turns out to be wrong would penalize heirs who did nothing wrong and destabilize every partition that later turns up an error involving just one person. By confining the nullity to the specific person wrongly included, Article 1105 protects the reasonable expectations of the actual heirs, who relied on the partition being final as to their own shares, while still correcting the mistake as to the person who had no right to inherit at all.
What heirs should do when this happens
If a partition already carried out is found to have included someone who was never truly an heir, the practical next step is to determine how much that person received and work out how it should be redistributed among the rightful heirs, rather than assuming the entire division must be redone from scratch. Because the shares of the genuine heirs remain valid under Article 1105, heirs facing this situation are usually correcting one specific allocation, not reopening the whole estate settlement.