Generally speaking the “defaut” or “none” material in Unreal looks like a grey checker board material. This is used to represent at least two conditions:
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A mesh that has no material applied from the incoming model (ex: make an object in max, don’t apply material, import → you’ll see the pattern)
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A temporary display while Unreal waits for the real material to compile on the graphic card. In that case, if this persists this is usually because one material applied on the object has compile errors, invalid connection etc.
Its difficult to help you more without precise information about your model, the steps you are doing in max prior to reimport etc. Since 4.21 has been shipping I would start by trying out the same workflow in that release. If it persists, i’d log a support ticket and provide all the data (archived max model, some screenshots documenting the issue etc)