When you enter "split" : To close 'split' it is always written 'split' instead of 'close split'. This occurs in French translation ; I don't know in other languages. In french : lorsqu'on clique sur 'scinder' puis que l'on veut fermer la fenêtre, on a toujours 'scinder' au lieu de 'fermer'.
Can't reproduce with git master, properly says "Scinder" and "Fermer", can you try running it with LANGUAGE=en_US and see if "Split" and "Close" work there?
I have tried with English language : We have "Split" and "Split" (but not "Close")
Confirmed in git master: "The button still says "Split" when it should say "Close Split"
here with master i have Split and Close
In the German version of Dolphin 17.12.3 (Kubuntu 18.04) the bug is present: Both instances have "Teilen" (split!), where it should be "Schließen" (or something similar) when the split view is activiated.
I might have something to do with multiple upgrading over the years. My laptop with one upgrade from 16.04 to 18.04 correctly says "Teilen" (split) and "Schließen" (close). My PC with multiple upgrades of all kinds (from one version to the next; then, the latest, from 16.04 to 18.04) displays this bug with both states showing a button "Teilen" (split) when it should be "Schließen" in double-window mode.
... sorry, this is for KDE: so 16.04 and 18.04 are, of course, Kubuntu versions.
I also see 'Split' and 'Close' as toolbar and menuitem texts. Tooltips change accordingly, too. Dolphin master and no translations installed.
I see this is fixed at least by Dolphin 19.08.1, shipped with Debian Testing. For old systems, either consider installing through backports, or upgrading to a later version.