transparent page.bgcolor rendering in the embedded browser is abnormal

Description

I use the Web Browser plugin to build a web GUI in the Unreal Engine game. I need to set the page.bgcolor to transparent to make the GUI page float on the 3D scene. However, when I updated the flet-fastapi from 0.19.0 to 0.20.2, the rendering of the webpage GUI became abnormal, and it seemed to be repeatedly re-rendering wrong things. I think the main problem is that the Chrome browser kernel version of Unreal Engine Web Browser plugin is older, but I can't expect Epic to quickly improve its kernel version, so can the flet keep as good adaptation as 0.19.0 version to the old browser kernel version?

My client_user_agent is Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Game/++UE5+Release-5.3-CL-29314046 UnrealEngine/5.3.2-29314046+++UE5+Release-5.3 Chrome/90.0.4430.212 Safari/537.36

Code example to reproduce the issue:

page.bgcolor = ft.colors.TRANSPARENT
# then use an old kernel Chrome/90.0.4430.212 open it in webpage

Describe the results you received:
The last shown dialog not fully removed while new controls not fully updated.

Describe the results you expected:
fully transparent background page as flet-0.19.0 already done

Additional information you deem important (e.g. issue happens only occasionally):

Flet version (pip show flet):

Name: flet
Version: 0.20.2
Summary: Flet for Python - easily build interactive multi-platform apps in Python
Home-page: 
Author: Appveyor Systems Inc.
Author-email: hello@flet.dev
License: Apache-2.0
Location: c:\users\yipin\anaconda3\envs\py310\lib\site-packages
Requires: cookiecutter, flet-runtime, packaging, qrcode, watchdog, websocket-client, websockets
Required-by: flet-fastapi

Give your requirements.txt file (don't pip freeze, instead give direct packages):

(The requirements)

Operating system:
Windows

Additional environment details:

No source file or test is named. Reproduce the issue with Flet 0.20.2, page.bgcolor set to transparent, and Unreal Engine Web Browser on Chrome 90, then compare with Flet 0.19.0. Done means the transparent page renders without leaving stale dialogs or partially updating controls.