The default parts have their own, un-editable, unmovable UVs when loading project files. Regardless of whether a part has an [H], it will disappear when loaded again. Any file saved in 0.1.0a can have this issue. This issue is present across at least 1.16 and 1.17. This can be temporarily solved by converting vanilla parts, and then deleting the parts that appear from this, but this is undone whenever the project file is loaded in the editor.