Add-in silently fails to install on Mac Outlook - outlook

Question

I am developing an add-in for Outlook. My manifest installs without problems in Outlook for the web, but when I try to install the manifest in Outlook for Mac (v16.43), it appears to succeed, but after closing the add-ins window and re-opening it, the add-in has disappeared.
I have generated some logs from Outlook, and see this error:
04/12/2020 14:29:13 Unexpected Manifest Issue encountered while parsing manifest,
add-in ID : <id>, Location : Error at element "Action" :
Add-in manifest parsing encountered an unexpected child node, Line=90, CharPosition=16
The relevant parts of the manifest are below. The Action element referred to in the error is the one in the MobileFormFactor section. If I remove the MobileFormFactor section, then the manifest installs successfully.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<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" xmlns:mailappor="http://schemas.microsoft.com/office/mailappversionoverrides/1.0" xsi:type="MailApp">
...
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
<Description resid="residDescription"></Description>
<Requirements>
<bt:Sets>
<bt:Set Name="Mailbox" MinVersion="1.3"/>
</bt:Sets>
</Requirements>
<Hosts>
<Host xsi:type="MailHost">
<DesktopFormFactor>
<ExtensionPoint xsi:type="AppointmentOrganizerCommandSurface">
<OfficeTab id="TabDefault">
<Group id="apptComposeGroup">
<Label resid="residDescription"/>
<Control xsi:type="Button" id="insertMeetingButton">
<Label resid="addLabel"/>
<Supertip>
<Title resid="addLabel"/>
<Description resid="residTooltip"/>
</Supertip>
<Icon>
<bt:Image size="16" resid="icon-16"/>
<bt:Image size="32" resid="icon-32"/>
<bt:Image size="48" resid="icon-48"/>
<bt:Image size="64" resid="icon-64"/>
<bt:Image size="80" resid="icon-80"/>
</Icon>
<Action xsi:type="ShowTaskpane">
<SourceLocation resid="residTaskpaneUrl"/>
</Action>
</Control>
</Group>
</OfficeTab>
</ExtensionPoint>
</DesktopFormFactor>
<MobileFormFactor>
<ExtensionPoint xsi:type="MobileOnlineMeetingCommandSurface">
<Control xsi:type="MobileButton" id="insertMeetingButton">
<Label resid="prodName"/>
<Icon>
<bt:Image size="25" scale="1" resid="icon-grey16"/>
<bt:Image size="25" scale="2" resid="icon-grey16"/>
<bt:Image size="25" scale="3" resid="icon-grey16"/>
<bt:Image size="32" scale="1" resid="icon-grey32"/>
<bt:Image size="32" scale="2" resid="icon-grey32"/>
<bt:Image size="32" scale="3" resid="icon-grey32"/>
<bt:Image size="48" scale="1" resid="icon-grey48"/>
<bt:Image size="48" scale="2" resid="icon-grey48"/>
<bt:Image size="48" scale="3" resid="icon-grey48"/>
</Icon>
<Action xsi:type="ShowTaskpane">
<SourceLocation resid="residTaskpaneUrl"/>
</Action>
</Control>
</ExtensionPoint>
</MobileFormFactor>
</Host>
</Hosts>
...
</VersionOverrides>
</VersionOverrides>
</OfficeApp>