[lorenzfx]

It really is my pet peeve with fish, but the last command that started with what you typed cannot be completed that way. For that, you need to use right-arrow.

I kept a fork of fish for that purpose for some time, but that was really too much hassle and I went back to zsh.

Which is pretty sad, because otherwise fish is amazing.

[NegativeLatency]

ctrl + e or end will complete commands in that case

[sixstringtheory]

IIUC, right-arrow also does, and option-right-arrow will complete individual arguments from the last command one by one (macOS).