I sourced test-classic model from snapd's asserts/sysdb/generic.go file. It's what ubuntu-image uses if a model is not provided for a classic image. I used this exact same file on amd64 and this behavior did not appear. Here are the contents:

```
type: model
authority-id: generic
series: 16
brand-id: generic
model: generic-classic
classic: true
timestamp: 2017-07-27T00:00:00.0Z
sign-key-sha3-384: d-JcZF9nD9eBw7bwMnH61x-bklnQOhQud1Is6o_cn2wTj8EYDi9musrIT9z2MdAa

AcLBXAQAAQoABgUCWYuXiAAKCRAdLQyY+/mCiST0D/0XGQauzV2bbTEy6DkrR1jlNbI6x8vfIdS8
KvEWYvzOWNhNlVSfwNOkFjs3uMHgCO6/fCg03wGXTyV9D7ZgrMeUzWrYp6EmXk8/LQSaBnff86XO
4/vYyfyvEYavhF0kQ6QGg8Cqr0EaMyw0x9/zWEO/Ll9fH/8nv9qcQq8N4AbebNvNxtGsCmJuXpSe
2rxl3Dw8XarYBmqgcBQhXxRNpa6/AgaTNBpPOTqgNA8ZtmbZwYLuaFjpZP410aJSs+evSKepy/ce
+zTA7RB3384YQVeZDdTudX2fGtuCnBZBAJ+NYlk0t8VFXxyOhyMSXeylSpNSx4pCqmUZRyaf5SDS
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nRaz6VBm09RJTuwUT6vNugXSOCeF7W3WN1RHJuex0zw+nP3eCehxFSr33YrVniaA7zGfjXvS8tKx
AINNQB4g2fpfet4na6lPPMYM41WHIHPCMTz/fJQ6dZBSEg6UUZ/GiQhGEfWPBteK7yd9pQ8qB3fj
ER4UvKnR7hcVI26e3NGNkXP5kp0SFCkV5NQs8rzXzokpB7p/V5Pnqp3Km6wu45cU6UiTZFhR2IMT
l+6AMtrS4gDGHktOhwfmOMWqmhvR/INF+TjaWbsB6g==
```

I also tried a different custom model, but saw the same behavior. The contents of the custom model are: 

```
type: model
authority-id: Pp6hQz8bUzbsiL3hRvxZ22lrCDlsKJAJ
series: 16
brand-id: Pp6hQz8bUzbsiL3hRvxZ22lrCDlsKJAJ
model: ubuntu-raspi-classic
architecture: arm64
base: core18
classic: true
display-name: Ubuntu Raspi Classic
distribution: lunar
grade: dangerous
snaps:
  -
    default-channel: latest/stable/ubuntu-23.04
    id: 3wdHCAVyZEmYsCMFDE9qt92UV8rC8Wdk
    name: firefox
    type: app
  -
    default-channel: latest/stable/ubuntu-23.04
    id: lATO8HzwVvrAPrlZRAWpfyrJKlAJrZS3
    name: gnome-42-2204
    type: app
  -
    default-channel: latest/stable/ubuntu-23.04
    id: gjf3IPXoRiipCu9K0kVu52f0H56fIksg
    name: snap-store
    type: app
  -
    default-channel: latest/stable/ubuntu-23.04
    id: IrwRHakqtzhFRHJOOPxKVPU0Kk7Erhcu
    name: snapd-desktop-integration
    type: app
  -
    default-channel: latest/stable/ubuntu-23.04
    id: jZLfBRzf1cYlYysIjD2bwSzNtngY0qit
    name: gtk-common-themes
    type: app
  -
    default-channel: latest/stable
    id: EISPgh06mRh1vordZY9OZ34QHdd7OrdR
    name: bare
    type: base
  -
    default-channel: latest/stable
    id: amcUKQILKXHHTlmSa7NMdnXSx02dNeeT
    name: core22
    type: base
  -
    default-channel: latest/stable
    id: PMrrV4ml8uWuEUDBT8dSGnKUYbevVhc4
    name: snapd
    type: snapd
timestamp: 2021-11-24T10:40:41+00:00
sign-key-sha3-384: SNdXhT977udsZN6CWRN7cTuSUGKhbXbID6iBMyykLzHBxYhQTUj6Jk9FDumLPUBe

AcLBcwQAAQoAHRYhBG53C20bnafIZaAIpl3EKJzwBddxBQJkG2ewAAoJEF3EKJzwBddxbQIP+QEC
Bjqznx4jmcIUFXc2F0xKl++JaoZhcArNSfEMa4CPemvn589JL5nAytQI2zNDjCPV2Os51tqXBUMx
nUHlqgOAAf9QtwHJ6i01nRAz/i7T07c1Inj5ixaF/KKEwPwF7dDEcCPOkxmQz9Nb8kmGY/XtM57E
vtQ2s5G9lfRc29DjQ7LgPA3jmS8ttS3EJ5ZH6wjNqM4jjxfT+wqB2ZSl8S28QR42dc/84puRqqai
6PZ36Id/nc71/k7oh6Yyq5xM5OhZwq3JaSS1T+bJi5mssCV+1RUeNb50iivJ6u8p7uGGd5psJkna
8Rne7cQDqeZjeYjtZh8jCckMdXNsEhTH/brlFyOy8m3ITDdC+4vU/q4bBltWATaN87ypG+4Cz9ft
KlZ4eCzNwKeJzdAdy9XrLxdICf8NwhbfgMGeqnySsUO1iWTQOK/1XNb7QA8zvbSOrrdn4PMpdcV6
ZioIY0MrC9kn1W8hGTPuBOQuTlmqtSJGViRejQQQSxPNe8L+i4vlcgGqCh8d8+qOteKjbJSZAqrl
ZJUul0dFQqEVzBGEiXCz8LnELfqnyJt22zQMIfFWhDiEKZUllkAtIvb5UO3nK15Z6Sh7MQVE9rDe
t0EE/SuS2y2/90ms14OrA6KVmhRlZd7uCx18vwCpOoeloNKa+kYlT9psbfImYgWpC1b2rfQr
```