Old stinky fish

Fish shell doesn’t work well with powerline. Turns out fish 2.4 is very outdated (Nov. 2016) and buggy.
It should be updated. Are those packages straight from raspbian or do you have influence?

It comes from the Raspbian Stretch repo. Since Raspbian does not provide a backports repo, no chance to install a newer version APT-wise.

You could go with the Buster testing image that we will soon ship as default: https://github.com/MichaIng/DietPi/issues/2935
That provides fish v3.0.2.

Is the upgrade to Buster going to be seemless?

This highly depends on what you have installed/configured. Generally it is not recommended as long as you don’t have some system and console experience to fix things. If you want to take the risk, do a full SDcard clone at best and follow these steps: https://dietpi.com/forum/t/qbittorrent-version-shipping-with-dietpi/2326/9