I am using a Nvidia Quadro K6000, I cannot run heightfields and heightfield_noises with Houdini 20. So I am using Houdini 19.5???

Graphics card problems disappeared with version 20: no explanation. I changed nothing on my computer or settings.

It is very unstable, I cannot see my heightfield anymore, I can just work with the mesh version I did from it. I used vdb to do mask. Could it be a reason for it? Except when doing flow, Heightfield are float arrays.

Which OS are you using? Are your drivers up to date? If you are using Linux, make sure that you're using the proprietary Nvidia drivers and not the Nouveau drivers.