I've had this issue happen occasionally on my end even when not using external texture or resource packs.
This mainly seems to be an issue regarding the implementation of Render Dragon as part of Minecraft Bedrock 1.16.200 and versions released after that build with how textures for models and menus are loaded.
I find that this issue does not occur on older versions of the game like Minecraft 1.16.40.
I believe this is due to how Render Dragon relies on the HLSL shading language (High-Level Shading Language) rather than the GLSL shading language (OpenGl Shading Language) used in previous versions which is also why Render Dragon is able to take advantage of hardware accelerated ray-tracing found in newer generation consoles and graphics cards.
Either way, It is quite strange and it is an issue that I am not entirely certain about.