Office.js content add-in insertion behavior

When I installed an Office.js content add-in in Excel/PowerPoint before 4 October 2017, it automatically added an "insert button" to the Insert tab on the ribbon. If I pressed this button, the content add-in itself is inserted to the current document. However on 4 October 2017 this behavior suddenly changed and when I now install a content add-in, the "Add" button on the install screen immediately inserts the content add-in itself into the document, without creating its own button on the ribbon.

Our content add-in's manifest is almost the same as the original manifest that is generated by Visual Studio and it does not matter whether the content add-in is installed from Office Store or a local shared folder. This phenomenon also affects our older product releases as well as other third party content add-ins.

My current Office version is Version 1708 (Build 8431.2094), but I don't think it is an Office version-related issue as I rolled back to a previous version before 4 October, but it didn't help at all.

I know that content add-ins can also be chosen from the "My Add-ins" dropdown list from Add-ins group on Insert tab, but that would be a step back in UX compared to a dedicated insert button on the ribbon.

Is it an intended new behavior of inserting content add-ins or is this a bug? Can this feature be controlled by developers? Is there any way to get our insert button back? Is there any way to put an "insert content add-in button" to a custom tab that is defined for task panes?

UPDATE:

Here is the manifest we use for the content add-in:

<?xml version="1.0" encoding="UTF-8"?>
<OfficeApp
          xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"
          xsi:type="ContentApp">

  <Id>xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx</Id>
  <Version>1.0.0.0</Version>
  <ProviderName>Xxxxxxxxxx</ProviderName>
  <DefaultLocale>en-US</DefaultLocale>
  <DisplayName DefaultValue="Insert stuff" />
  <Description DefaultValue="Insert stuff to current document"/>
  <IconUrl DefaultValue="https://xxxxxxx/icon.png" />
  <SupportUrl DefaultValue="http://xxxxxxxx" />
  <AppDomains>
    <AppDomain>http://localhost:12345</AppDomain>
  </AppDomains>
  <Hosts>
    <Host Name="Presentation" />
    <Host Name="Workbook" />
  </Hosts>
  <DefaultSettings>
    <SourceLocation DefaultValue="http://localhost:12345/path/to/index.html" />
    <RequestedWidth>400</RequestedWidth>
    <RequestedHeight>440</RequestedHeight>
  </DefaultSettings>
  <Permissions>ReadWriteDocument</Permissions>
</OfficeApp>