Not that many hashmaps see hundreds of millions of entries in their lifetime. Given that Zig isn’t widely used, it’s probable that noone has really stumbled upon this behaviour in a non-benchmark setting.
Now that can happen with every compiler but using one in still in alpha significantly increases the risk.
The problem is, of course, you almost certainly didn't test anything near 100% and probably not even everything users may do with your software, so it's a risky bet.
Conversely, taking on a dependency prior to its 1.0 just means you're on the hook for dealing with API changes as they come up. But as long as the API you expose doesn't change, it's fine to be on 1.0 yourself.
[0]: https://github.com/ziglang/zig/issues?q=is%3Aopen+is%3Aissue...