GPG shows my own UID as [ full ] after re-importing signatures—even though it was [ ultimate ] when freshly generated

In GPG, my own uid presents as [ full ] rather than [ ultimate ].

gpg --list-keys --with-sig-check

pub   ed25519 2025-07-02 [SC] [expires: 2030-07-01]
      GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
uid           [  full  ] Me 
sig!3        XXXXXXXXXXXXXXXX 2025-07-02  [self-signature]
sig!3        XXXXXXXXXXXXXXXX 2025-07-04  [self-signature]
sig!         YYYYYYYYYYYYYYYY 2025-07-03  Person1
sig!         ZZZZZZZZZZZZZZZZ 2025-07-03  Person2
sub   cv25519 2025-07-02 [E] [expires: 2030-07-01]
sig!         XXXXXXXXXXXXXXXX 2025-07-02  [self-signature]
sub   ed25519 2025-07-02 [S]
sig!         XXXXXXXXXXXXXXXX 2025-07-02  [self-signature]

This is pretty weird. If I delete the uid

adduid
uid 2
primary
uid 2
uid 1
deluid

and recreate it:

adduid
uid 2
primary
uid 2
uid 1
deluid

I end up with

gpg --list-keys --with-sig-check

pub   ed25519 2025-07-02 [SC] [expires: 2030-07-01]
      GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
uid           [ultimate] Me 
sig!3        XXXXXXXXXXXXXXXX 2025-07-04  [self-signature]
sub   cv25519 2025-07-02 [E] [expires: 2030-07-01]
sig!         XXXXXXXXXXXXXXXX 2025-07-02  [self-signature]
sub   ed25519 2025-07-02 [S]
sig!         XXXXXXXXXXXXXXXX 2025-07-02  [self-signature]

but once I import Person1 and Person2's signatures, I'm back to [ full ]. What's going on here?