SetSubMesh #1 shares part of its index buffer with SubMesh #2. Sharing part of an index buffer can result in undefined behavior and will be deprecated soon. To future-proof your Project, fix the indexStart/indexCount
UnityEngine.Mesh:SetSubMesh (int,UnityEngine.Rendering.SubMeshDescriptor,UnityEngine.Rendering.MeshUpdateFlags)
ImGuiNET.Unity.ImGuiRendererMesh:UpdateMesh (ImGuiNET.ImDrawDataPtr,UnityEngine.Vector2) (at Packages/dear-imgui-unity/ImGuiNET.Unity/Renderer/ImGuiRendererMesh.cs:127)
ImGuiNET.Unity.ImGuiRendererMesh:RenderDrawLists (UnityEngine.Rendering.CommandBuffer,ImGuiNET.ImDrawDataPtr) (at Packages/dear-imgui-unity/ImGuiNET.Unity/Renderer/ImGuiRendererMesh.cs:71)
ImGuiNET.Unity.DearImGui:Update () (at Packages/dear-imgui-unity/ImGuiNET.Unity/DearImGui.cs:157)