It’s very likely specified as an acyclic graph (because, without checking, I see no reason why it would not be). So if you made a cyclic graph it would be wrong. It’s possible, likely even, that a lot of git’s implementation and associated tools don’t check for that, because it’s very hard[1] to achieve, but that just means stuff would break in more non-obvious ways.

[1] Constructing a cyclic git graph means finding a hash collision, and git uses SHA1 which is known to have weaknesses. With another hash algorithm it might be “practically impossible” instead of “very hard”.