If there is any ambiguity, such a seal should be fixed (and the document should be signed). However, discuss the matter your local contract attorney if considering this issue other than academically.
As to whether the seal is legitimate to begin with, I will provide a brief general discussion under US law (which varies by state even within the US). You would need to do a fairly careful analysis of the law in your jurisdiction, including law specific to the kind of document receiving a "seal."
There are a variety of places from which some seals might gain meaning. Ordinarily they would not have any special meaning, although if it is a custom or practice of a business to only use them on certain documents, that in itself could serve as evidence that those documents went through whatever process the company requires before sealing them. Its meaning in contract law in the United States would primarily be as evidence of that process (e.g. tending to show authenticity of the document, tending to show it was the final version, whatever requirements the company has as its standard business practice), although in some cases it could arguably be used as evidence of contract acceptance.
For example, for the sale of goods, the Uniform Commercial Code (a system of laws that each state modifies for itself and then adopts a version of) provides that
Unless otherwise unambiguously indicated by the language or
circumstances (a) an offer to make a contract shall be construed as
inviting acceptance in any manner and by any medium reasonable in the
UCC Section 2-206(1) (emphasis added).
However, often contracts provide specifically that they can only be modified by a signed writing. So if a pre-existing contract specified that, then modifying it with a seal would be unlikely to succeed. Similarly, important contracts are often governed by a state's variation on the Statute of Frauds. In the context of the sale of goods, for example,
(1) Except as otherwise provided in this section a contract for the
sale of goods for the price of $500 or more is not enforceable by way
of action or defense unless there is some writing sufficient to
indicate that a contract for sale has been made between the parties
and signed by the party against whom enforcement is sought or by his
authorized agent or broker.
UCC Section 2-201 (emphasis added).
However, the statute of frauds is ordinarily a matter of state law that varies slightly by state.
In many cases the statute of frauds might be an effective bar to contract enforcement. Traditionally it includes certain kinds of contracts where greater formalism is required, such as contracts for the sale of land, contracts lasting more than one year, contracts to guaranty a debt, and other contracts where the law wants to minimize the possibility of ambiguity and fraud. There are also some exceptions, and in many states they would include the case where the party to be held to the agreement admits "Yes, I stamped that document and only stamp documents where other people use a signature."
Realistically, a seal following a certain business practice may very, very occasionally be useful as evidence tending to show authenticity, but unless you find a specific authority for it under the law of your jurisdiction, it doesn't have the meaning of a notarial seal or other special legal meaning.
That being said, there are some cases where a state's law may include a company seal as a legitimate method of "signing" a document. The Uniform Commercial Code would probably allow a seal to be used as a way of endorsing a check, for example.
(b) A signature may be made (i) manually or by means of a device or
machine, and (ii) by the use of any name, including a trade or assumed
name, or by a word, mark, or symbol executed or adopted by a person
with present intention to authenticate a writing.
UCC Section 3-401