After looking at that video, it at least doesn’t feel like the current memory leaks of Dolphin reported on bugs.kde.org that I can find… so this might be a new bug.
https://bugs.kde.org/show_bug.cgi?id=485736
https://bugs.kde.org/show_bug.cgi?id=485868
Unless its related to bug 485868 in some way with certain icon theme packs causing a memory issue (might be others than the specific dark version of Flatery causing issues)… but I think that is a bit of a stretch considering you are using Hyprland. So doubtful something like that could cause it.