Building Virtual Texture yields unpredictive results

I am on Unity 2018.3 beta12 and the Oct 30 build of AT2 (but this problem has persisted for about a month, on different Unity 2018.3 betas)...
In the attached video, I rebuild a simple Virtual Texture several times, each time yielding different and strange results. It is a simple demo scene with just a light, a camera, a model and a Virtual Texture Manager using a new Virtual Texture with just a dozen textures (two or three materials).
https://www.dropbox.com/s/41u5cw6yrotnm ... 3.mp4?dl=0
Any idea what is going on?
(the popup windows displayed when Virtual Texture is rebuilt is not visible in the video - just nothing happens for some 30sec or so a couple of times)
In the attached video, I rebuild a simple Virtual Texture several times, each time yielding different and strange results. It is a simple demo scene with just a light, a camera, a model and a Virtual Texture Manager using a new Virtual Texture with just a dozen textures (two or three materials).
https://www.dropbox.com/s/41u5cw6yrotnm ... 3.mp4?dl=0
Any idea what is going on?
(the popup windows displayed when Virtual Texture is rebuilt is not visible in the video - just nothing happens for some 30sec or so a couple of times)