question

@Ashley Handscomb Retallack Yes, double-click on the .hip file and start houdini and you will get a problem with the material being lost. This also shows that htoa loading material is not successful.

All my input are based on this:
....There is always a problem with the htoa material. Just save the houdini file and open the houdini file. The materials of arnold are all lost
witch is NOT !

Double-clicking to open a file is another story!

In this way, it is no problem to open houdini first, and then go to file-Open to access the file. Can be opened successfully. This video also demonstrates this way to open. Now is a saved file, directly by double-clicking it will lose the material, you see my video is a bit boring, you insist on seeing me. You also demonstrated this in my video. You double-click on the saved .hip file to start the game. The opportunity to discard the material indicates that the houdini recognition material was unsuccessful at startup. Will cause a loss.https://vimeo.com/351411244

Here is a video showing the material after saving the scene
https://youtu.be/g4uNKUL67lc
and here is the video showing your scene without any problem
https://youtu.be/3M2I1ebUIfo
So are you satisfied?

Ok, there is no way to send houdini.evn I turned into txthtoa.zip

I never had that kind of problem.The material is the same after saving, never lost
The only confilt is with renderman.
Here is snapshot houdini shop and houdini mat

Again can you post your houdini.env file here?

I recorded a silent video and I understood it at a glance.https://vimeo.com/351411244

So the issue is with loading the scene by double-clicking the file in windows it to load houdini?

This is not the problem. Why do you always think of this kind of problem? The document that houdini always thinks about. The arnold path of the .evn file is wrong, or it conflicts with other ones! The first is even a conflict. How did these conflicts before? Second, even if you only install arnold without installing any other renderers, the material will not be the same and will be lost. Isn't there a real fundamental solution? Everything is simple. The blow can be tested directly, just install htoa and you will understand.

To avoid conflicts with other rendering engines,it is best to use json file for each renderer like
Arnold.json
Redshift.json
Vray.json
Corona.json
Renderman.json
etc..

Leave Houdini.env file empty, do not put anything in it

Create packages folder inside "C:\Users\Your_USER_NAME\Documents\houdini17.5" and put all your json file

#Here is one for Arnold.json
{
"env":
[
{
"PATH":
[
"C:/Users/rachid/htoa/htoa-4.2.0_rcd74057_houdini-17.5.327/htoa-4.2.0_rcd74057_houdini-17.5.327/scripts/bin;"
]
},
{
"HOUDINI_PATH":
[
"C:/Users/rachid/htoa/htoa-4.2.0_rcd74057_houdini-17.5.327/htoa-4.2.0_rcd74057_houdini-17.5.327"
]
},
{
"ARNOLD_PLUGIN_PATH":
[
"C:/Arnold_Shader/alShaders/bin;C:/Arnold_Shader/Octoplug/bin;C:/Arnold_Shader/pota/bin;C:/Arnold_Shader/dusty/bin;C:/Arnold_Shader/Osl/bin;"
]
}
]
}

########## Redshift.json
{
"env":
[
{
"PATH":
[
"C:/ProgramData/Redshift/bin;"
]
},
{
"HOUDINI_PATH":
[
"C:/ProgramData/Redshift/Plugins/Houdini/17.5.327"
]
}
]
}

########## Renderman.json
{
"env":
[
{
"RFHTREE":
[
"C:/Program Files/Pixar/RenderManForHoudini-22.6"
]
},
{
"RMANTREE":
[
"C:/Program Files/Pixar/RenderManProServer-22.6"
]
},
{
"PATH":
[
"C:/Program Files/Pixar/RenderManProServer-22.6/bin;"
]
},
{
"HOUDINI_PATH":
[
"C:/Program Files/Pixar/RenderManForHoudini-22.6/17.5"
]
}
]
}

The only conflict even with json.file is with Arnold/Renderman
The solution is simply load Arnold or Renderman (by renaming Arnold.json to something like Arnold.json_Backup or Renderman.json to Renderman.json_XX )
Hope this help