Interfaces missing from Device Type

[llamafilm]

I've started using the public Netbox demo server to test a custom API integration. I notice that sometimes (always?) a Device Type shows zero interfaces even though they should exist according to the Interfaces page. I'm not sure how this problematic state was created, or how to reproduce it on my own server. This demo server is currently running v3.7.0. Here's one example:
The API https://demo.netbox.dev/api/dcim/device-types/7/ shows "interface_template_count": 0. And the GUI page agrees: https://demo.netbox.dev/dcim/device-types/7/

But the interface-templates API shows 51 interfaces with this device type ID. https://demo.netbox.dev/api/dcim/interface-templates/?devicetype_id=7. Truncated output:

{
    "count": 51,
    "next": "https://demo.netbox.dev/api/dcim/interface-templates/?devicetype_id=7&limit=50&offset=50",
    "previous": null,
    "results": [
        {
            "id": 126,
            "url": "https://demo.netbox.dev/api/dcim/interface-templates/126/",
            "display": "GigabitEthernet1/0/1",
            "device_type": {
                "id": 7,
                "url": "https://demo.netbox.dev/api/dcim/device-types/7/",
                "display": "C9200-48P",
                "manufacturer": {
                    "id": 3,
                    "url": "https://demo.netbox.dev/api/dcim/manufacturers/3/",
                    "display": "Cisco",
                    "name": "Cisco",
                    "slug": "cisco"
                },
                "model": "C9200-48P",
                "slug": "c9200-48p"
            },
            "module_type": null,
            "name": "GigabitEthernet1/0/1",
            "label": "",
            "type": {
                "value": "1000base-t",
                "label": "1000BASE-T (1GE)"
            },
            "enabled": true,
            "mgmt_only": false,
            "description": "",
            "bridge": null,
            "poe_mode": null,
            "poe_type": null,
            "rf_role": null,
            "created": "2021-04-14T00:00:00Z",
            "last_updated": "2021-04-14T17:36:01.926000Z"
        },

After I add a new interface to this device type, now the device-types API https://demo.netbox.dev/api/dcim/device-types/7/ shows "interface_template_count": 1.

And the GUI shows all 52 interfaces:

[candlerb]

Looks to be the same problem as #14430 (comment) - except for device-template + interface-template rather than device + interface.

It's useful to know that the count is being initialized to zero: that would explain why the tabs are suppressed.