Android Question Webview html form submit issue

I have an old app (sdk version 28) had been running well as below for more than a year.

1. app sends parameters to backend server using codes below
...
Private wv_scanlocation As WebView
...
Dim lHttpJob As HttpJob ' use local resource to avoid crash
lHttpJob.Initialize(strjob, Me)
lHttpJob.PostString(gUrl, strparam)
...

Sub JobDone (Job As HttpJob)
If Job.Success = True Then
Select Job.JobName
...
Case "scanlocation"
wv_scanlocation.LoadHtml(Job.GetString)
...

2. backend server response with a data entry form with a submit button to the app screen
...
<form id=scanpickuppcs method=post action="https://xxxx.com/abc">
<table>
<tr><td>Seal <%=req("fld6")%> saved </td></tr>
<tr><td>No.of items pickuped: <input type=text name=fld7> </td></tr>
<tr><td><input class=btnSubmit type=submit name=submit value="Submit"></td></tr>
</table>
</form>
...
3. when user click Submit button. the data entered will submit to back end server

But recently I found the following issue
- in step 3, when user click, I could see Submit button is pressed down but no form data is sent to backend server

I tried to recompile app as version 30 but that did not solve the problem.

Any body has any hints to solve this? Thanks.