Steps to replicate the issue (include links if applicable):

When adding values to combobox field, saving the form, then edits the form again, the value replaced with 0
Here's minimal wikitext to check this

<noinclude>
    This form is being used to create my_page pages for use as Main pages in our system
    {{#forminput:form={{PAGENAME}}|button text=Create my page|placeholder=Page name}}
    
    </noinclude>
    <includeonly>
    <div id="wikiPreview" style="display: none; padding-bottom: 25px; margin-bottom: 25px; border-bottom: 1px solid #AAAAAA;"></div>
==Page==
    {{{for template|my_page}}}
   
    {{{field|parts 1|holds template}}}
   
    

 
    {{{end template}}}
    {{{for template|part 1|multiple|add button text=Add part|embed in field=my_page[parts 1]|display=table}}}
    {{{field|title_section|input type=combobox|label=part title (can be the target page name)|values from namespace=Main}}}
    {{{field|subtitle|label=part content}}}
    {{{end template}}}
    __NOTOC__ __NOEDITSECTION__

• create form with this wikitext
• create page with this form, add value to one or more section_title fields.
• Save it.
• Check in ?action=edit - value is OK
• Edit the page with the form.
• You dont have to change something
• Save it
• Check in ?action=edit
• section_title=0

What happens?:

value is becoming 0

What should have happened instead?:

value presists

Software version :

• MW 1.40.3
• PageForms commit: be96c99fe44dcaa2d4764ee3a8eaca47d443a339

Other information
The data I've mange to grab:

• this is PHP bug: the source code of HTML rendering input with value="0"
• while editing the page with the form (second time) $cur_value of the problematic field, for example in PFFormPrinter::formHTML is an array of [0 => 'FIELD_VALUE']. I don't have enough knowledge if this as intended or not.
• If I'm understanding well, this is part of $gridValues var