After carefully study, the root cause is identified:
When focus in input area, the soft keyboard will resize the webview.
When the ad banner displayed in split mode (overlap:false), it will also resize the webview to avoid overlap the web content.
If both resize the webview at the same time, this will cause conflict.
When soft keyboard display, it will always try to resize the existing view at bottom. If the banner ad displayed at bottom, it will give up resizing. As the soft keyboard code is from Android system, this issue cannot be resolved perfectly.
Here a few workaround methods are suggested to avoid the issue happen:
When you have some input box or textarea on UI, avoid show banner at bottom. You can either hide banner, or show banner at top.
Use overlap:true (as the banner will overlap web content, you need reserve some margin area for it.)