ghcup upgrade (0.1.5 -> 0.1.8) reports Success but does nothing

Here's a trimmed transcript of the shell session:

$ ghcup list                                                         2020-08-07 09:14:26 ba pts/10
[ Warn  ] New GHCup version available: 0.1.8. To upgrade, run 'ghcup upgrade'
[ Warn  ] New GHC version available: 8.10.1. To upgrade, run 'ghcup install 8.10.1'
   Tool  Version Tags                                 Notes
✗  ghc   7.10.3  base-4.8.2.0                              
✗  ghc   8.0.2   base-4.9.1.0                              
✗  ghc   8.2.2   base-4.10.1.0                             
✗  ghc   8.4.1   base-4.11.0.0                             
[------------------------snip-----------------------]
$ ghcup upgrade                                                      2020-08-07 09:14:30 ba pts/10
[ Warn  ] New GHCup version available: 0.1.8. To upgrade, run 'ghcup upgrade'
[ Warn  ] New GHC version available: 8.10.1. To upgrade, run 'ghcup install 8.10.1'
[ Info  ] Upgrading GHCup...
[ Info  ] downloading: https://downloads.haskell.org/~ghcup/0.1.8/x86_64-linux-ghcup-0.1.8
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 7214k  100 7214k    0     0  21.4M      0 --:--:-- --:--:-- --:--:-- 21.4M
[ Info  ] verifying digest of: ghcup
[ Info  ] Successfully upgraded GHCup to version 0.1.8
$ ghcup list                                                         2020-08-07 09:14:37 ba pts/10
[ Warn  ] New GHCup version available: 0.1.8. To upgrade, run 'ghcup upgrade'
[ Warn  ] New GHC version available: 8.10.1. To upgrade, run 'ghcup install 8.10.1'
   Tool  Version Tags                                 Notes
✗  ghc   7.10.3  base-4.8.2.0                              
✗  ghc   8.0.2   base-4.9.1.0                              
[------------------------snip-----------------------]

Here's the output of debug-info:

$ ghcup debug-info                                                   2020-08-07 09:14:39 ba pts/10
[ Warn  ] New GHCup version available: 0.1.8. To upgrade, run 'ghcup upgrade'
[ Warn  ] New GHC version available: 8.10.1. To upgrade, run 'ghcup install 8.10.1'
Debug Info
==========
GHCup base dir: /home/bardur/.ghcup
GHCup bin dir: /home/bardur/.ghcup/bin
GHCup GHC directory: /home/bardur/.ghcup/ghc
GHCup cache directory: /home/bardur/.ghcup/cache
Architecture: amd64
Platform: Linux UnknownLinux
Version: v0.1.5

It might be of interest that I'm using symlinks to manage different versions of things, so ~/bin/ghcup (which is the version in my PATH) is a symlink to ~/stow/ghcup/bin/ghcup which is itself a symlink to ~/stow/ghcup/bin/ghcup-0.1.5 (the binary I originally downloaded).

Now, I gather by this that I'm probably trying to tilt at windmills by having multiple versions of ghcup itself (which I will give up on), but AFAICT ghcup definitely shouldn't be reporting sucess in this case.