A non-manifold issue along the grid-plane

So I’ve created this piece of geometry and for the 2nd time I find this non-manifold region which appears to be exactly on the grid-plane in the 3D view area.
I’d really like to figure out what I’m doing while creating the geometry that creates this issue.
Any ideas please.

I created the geometry using a cylinder, rotating and scaling it, some extruding and insetting.
I deleted half the geometry vertically along the z axis and once complete I mirrored it.
There isn’t anywhere I’m aware I did anything along the horizontal axis to create this mess between the part above and the part below the grid. Maybe a setting I’m missing?
Thanks in advance.

You did nothing wrong. You just did not go far enough and did not do the last required step.
You have to merge by distance vertices of one half with overlapped vertices of mirrored half.

That will not happen automatically unless you enable automerging before mirroring.

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Thanks zeauro, but the problem with that is that I mirrored the ring through the vertical axis, in other words along the selection in this image.

So I don’t understand why a disconnect between the top and bottom occurred.