Grub doesn't recognize linux-surface kernel

I have a Surface Pro 6 with Ubuntu Studio (low-latency kernel) installed.

$ uname -a
Linux ctrl 5.11.0-38-lowlatency #42~20.04.1-Ubuntu SMP PREEMPT Tue Sep 28 21:54:36 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

To install the linux-surface kernel I followed the steps described here. The kernel is installed under /boot next to the off-the-shelf low-latency kernel:

$ ls -lisha | grep vmlinuz
14155785    0 lrwxrwxrwx  1 root root   28 Okt 19 18:35 vmlinuz -> vmlinuz-5.11.0-38-lowlatency
14156103 9,8M -rw-------  1 root root 9,8M Sep 28 17:39 vmlinuz-5.11.0-38-lowlatency
14155810 8,5M -rw-r--r--  1 root root 8,5M Okt 20 21:45 vmlinuz-5.14.13-surface
14156100 8,5M -rw-r--r--  1 root root 8,5M Okt 27 22:59 vmlinuz-5.14.14-surface
14155780    0 lrwxrwxrwx  1 root root   23 Okt 23 16:26 vmlinuz.old -> vmlinuz-5.14.13-surface

Secure Boot is activated the the root key is properly installed

$ mokutil --sb-state 
SecureBoot enabled
$ mokutil --list-enrolled
#...
[key 2]
SHA1 Fingerprint: 05:f6:aa:10:9c:1c:62:67:1e:75:bd:75:d3:d0:51:47:20:63:c0:81
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            43:3e:21:a6:6d:1a:2a:a6:84:02:e2:07:2d:af:4e:41:93:6a:23:3e
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=XX, L=Default City, O=Default Company Ltd, CN=linux-surface
        Validity
            Not Before: Jan 20 21:08:24 2020 GMT
            Not After : Jan 17 21:08:24 2030 GMT
        Subject: C=XX, L=Default City, O=Default Company Ltd, CN=linux-surface
#...

Yet, when I update grub the linux-surface kernel is not respected and will not show up in the grub menu. A similar question was previously asked here. But before adding a custom menu entry to grub I'd rather understand why update-grub ignores the linux-surface kernel and find a more persistent solution than a custom grub menu entry.