this is a known issue with convert to bezier path.
After you convert the rectangle to a bezier path select the pen tool, hold down the Ctrl/Cmnd tool to change from the add point to the direct selection tool, drag the selection over the upper left bezier point, right click and choose Mask and Shape Path > Set First Vertex.
next time when you create a shape and you know you want the bezier vertices, just click Alt as you drag if from the rectangle tool and you will get vertices instead of parametric shape
Sorry, everything is just fine. First time I changed it only on one keyframe, now made new keyframes after setting first vertex. Thank you very much!
this should work if you follow the exact steps. you do this before you create your animation otherwise you would have to set first vertex for each kayframe. but instead of messing around with converting to bezier path and doing the first vertex workflow when you already know you want the path vertices and not the parametric properties, just start over by using a parametric shape with the Alt key and be done with it:
1. create your parametric shape using the alt key as you drag it.
2. create your path keyframes.
You need the same number of points in both states. Your points are going to move in a straight line to matching points.
I would do this kind of thing in Illustrator using the blend tool, then expand the blend and release to layers. Then import as an AE comp and you will have exactly what you want. Take a look at this quick tutorial.
If fixing your path by having the same number of vertices and appropriately marking the first vertex does't do the trick then try the morph in Illustrator technique. I use it all the time for morphs that last up to 250 or so frames.