Outlook add-in ribbon icon doesn't show up only on Outlook app for Windows

Question

I'm developing an add-in for Outlook to be used on the Appointments tabs. My issue is that the ribbon icon that initializes the add-in doesn't display the proper icon (ends up falling back to the default blue hexagon from Microsoft) but only on Windows Outlook apps. The icon shows up properly on Outlook for Mac and on the Outlook web clients.

Tried already resizing the icons to 64x64 and 128x128 (high res), removing and reinstalling the add-in, and validating through the manifest validator. All seems fine but the issue persists.

These are parts of the manifest (URL's replaced with localhost) that I judge relevant to the issue (Please feel free to ask for other sections if I missed anything.

<IconUrl DefaultValue="https://localhost:8080/assets/icon-dark-64.png" />
<HighResolutionIconUrl DefaultValue="https://localhost:8080/assets/icon-dark-128.png" />

<DesktopFormFactor>
    <FunctionFile resid="functionFile" />
    <ExtensionPoint xsi:type="AppointmentOrganizerCommandSurface">
        <OfficeTab id="TabDefault">
            <Group id="meetingGroup">
                <Label resid="groupLabel" />
                <Control xsi:type="Button" id="meetingOpenPaneButton">
                    <Label resid="paneMeetingButtonLabel" />
                    <Supertip>
                        <Title resid="paneMeetingSuperTipTitle" />
                        <Description resid="paneMeetingSuperTipDescription" />
                    </Supertip>
                    <Icon>
                        <bt:Image size="16" resid="icon16" />
                        <bt:Image size="32" resid="icon32" />
                        <bt:Image size="64" resid="icon64" />
                        <bt:Image size="80" resid="icon80" />
                    </Icon>
                    <Action xsi:type="ShowTaskpane">
                        <SourceLocation resid="messageMeetingTaskPaneUrl" />
                    </Action>
                </Control>
            </Group>
        </OfficeTab>
    </ExtensionPoint>
</DesktopFormFactor>

<Resources>
    <bt:Images>
        <bt:Image id="icon16" DefaultValue="https://localhost:8080/assets/icon-dark-16.png" />
        <bt:Image id="icon32" DefaultValue="https://localhost:8080/assets/icon-dark-32.png" />
        <bt:Image id="icon64" DefaultValue="https://localhost:8080/assets/icon-dark-64.png" />
        <bt:Image id="icon80" DefaultValue="https://localhost:8080/assets/icon-dark-80.png" />
    </bt:Images>

    ... <!-- Other resources -->
</Resources>

Answer 1

All image URIs, such as those used for add-in commands, must support caching. The server hosting the image should not return a Cache-Control header specifying no-cache, no-store, or similar options in the HTTP response. Find more details here.