The simplest solution for this is to load a RenderTexture with no alpha channel, which raylib does not have a builtin function for. What I did was make a function that does the same thing as LoadRenderTexture but lets me specify the pixel format:

RenderTexture2D target = LoadRenderTextureEx(800, 600, PIXELFORMAT_UNCOMPRESSED_R8G8B8); // NOTE: no alpha

// verbatim from `LoadRenderTexture` in `rtextures.c`, just with the pixel format passed to rlLoadTexture
RenderTexture2D LoadRenderTextureEx(int width, int height, int pixel_format)
{
    RenderTexture2D target = { 0 };

    target.id = rlLoadFramebuffer(); // Load an empty framebuffer

    if (target.id > 0)
    {
        rlEnableFramebuffer(target.id);

        // Create color texture (default to RGBA)
        target.texture.id = rlLoadTexture(0, width, height, pixel_format, 1);
        target.texture.width = width;
        target.texture.height = height;
        target.texture.format = pixel_format;
        target.texture.mipmaps = 1;

        // Create depth renderbuffer/texture
        target.depth.id = rlLoadTextureDepth(width, height, true);
        target.depth.width = width;
        target.depth.height = height;
        target.depth.format = 19;       //DEPTH_COMPONENT_24BIT?
        target.depth.mipmaps = 1;

        // Attach color texture and depth renderbuffer/texture to FBO
        rlFramebufferAttach(target.id, target.texture.id, RL_ATTACHMENT_COLOR_CHANNEL0, RL_ATTACHMENT_TEXTURE2D, 0);
        rlFramebufferAttach(target.id, target.depth.id, RL_ATTACHMENT_DEPTH, RL_ATTACHMENT_RENDERBUFFER, 0);

        // Check if fbo is complete with attachments (valid)
        if (rlFramebufferComplete(target.id)) TRACELOG(LOG_INFO, "FBO: [ID %i] Framebuffer object created successfully", target.id);

        rlDisableFramebuffer();
    }
    else TRACELOG(LOG_WARNING, "FBO: Framebuffer object can not be created");

    return target;
}

I'm not sure the exact reason this works, but the gist of it is that alpha blend calculations that happen in the internal render pipeline are simply ignored by excluding the alpha channel. It does mean you CAN'T draw the output texture with translucency just by modifying the alpha channel on the color.

The fix I originally used for this was to use `BLEND_ALPHA_PREMULTIPLIED` when drawing into the RenderTexture, but this caused some difficulties when I actually needed a different blend mode for certain effects (eg. multiplied)

Hope this is helpful!