I created a two variants of same command:
A) /admin skin [targetPlayerId] [skinId]
B) /admin skin [skinId]

All parameters are int.

When the server loads in the console log I see both variants are loaded:

[SampSharp:DEBUG] Registering command /admin skin [skinId]
[SampSharp:DEBUG] Registering command /admin skin [targetPlayerId] [skinId]

In game if I write /admin skin the server prints Usage: /admin skin [skinId]
If I write /admin skin 33 it prints Usage: /admin skin [targetPlayer] [skinId]
If I write /admin skin 1 33 then it works OK with the "A" variant.
It is impossible to use the "B" variant.

The commands are declared as suggested here: https://sampsharp.net/player-commands#grouping-commands
I'm not using permissions nor overriding the default commands behavior.
I tried to swap the order of the command declaration, it swapped the order of the command in the server load log, but it was not changed the behavior.