The x and z axes have a different origin.

[Max N]

The x and z axes have a different origin.

The planes are offset from each other. Isometric drawing is impossible, if only every time not to move the entire image.
Maybe I'm doing something wrong?

 

 

[JET_Affinity]

Just by way of explanation…

Have you ever wondered why isometric drawing can be done with a single grid, but any other orientation (dimetric or trimetric) requires three grids? That's because, by definition, all three axes in isometric are equally foreshortened. The same measuring scale works for all three. So in effect, "all three grids" and their increments are superimposed precisely on top of each other in an isometric orientation…but not in any other.

That's one of the problems inherent to grids-based approaches to axonometric drawing. In your example, you've drawn a square on one grid. The other two perpendicular grids don't know which corner of that square you currently consider "the origin." Continually moving the artwork as in your screen clip is, of course, unacceptable. So using grids, you have to be able to continually "reset" the origin to whatever point of the drawing you at the moment consider the "origin" of your next measure. Doing that needs to be as fluid and instant and natural as possible. Since emulating pre-computer physical media is usually the goal of user interface design, It needs to be at least as fluid and instant and natural as it was before computers. But it's not.

In the pre-computer days of productive isometric drawing "on the board," there were no "grids" on our drafting tables. There were (appropriately) just axes. (It's not called axonometric drawing for nothing.) The "axes" were the ruler scale(s) attached to the head of the track drafter, which you held in the palm of your left hand (if you were right-handed), and thereby fluidly glided to any point of interest in the drawing while maintaining the same angle. The illustrator effectively and intuitively "zeroed" the origin of the appropriate axis each time he started a new measure, pretty much without even thinking about it. Grids just don't emulate that very well.

For flat graphic page design, a designer cares about the orientation of a spacing grid relative to some corner of the page. But for illustration, an illustrator doesn't care one whit about the "origin" of a grid relative to some corner of the page; his measures are made along the angles represented by the grids, but must be zeroed at any current point of interest in the drawing.

(I don't have Affinity installed on this machine, so I'll leave it to someone else to tell you how to do that.)

JET

 

[Ben]

You just need to enable "Show axis editing handles" on the Grid and axis manager.  You'll then be able to see the grid origin widget, which will allow you to quickly set the origin intersection of the three planes.

 

Beyond that - as @JET_Affinity says - you'll need to get your head around why non-iso axonometric grids only have one common point between grids for the planes.  If considering a cube - you'd need to place the origin at the most prominent point of the object.  As you cycle planes - that point will remain constant.

 

In future the axonometric features will be improved.