I am using latest Mac OS 13.4 and JetBrains Fleet 1.19.95. Maybe it's just me, but delete button don't works for me with any of the keymaps available – I receive just beeping.
I tried editing default keymaps, but receive only beeping or wrong behaviour: backwards deleting when I don't expect it, etc.
UPD: I made some additional investigation and deleting left is called delete backward, but to delete selected text I need to use delete forward. Also, for some reason on Mac OS there is delete button shortcut that is directed right, I don't even know which button is it! Plain delete button gives delete directed to the left. Maybe someone has keymap for Fleet working appropriately on Mac OS and can share it?
Please check if there are any conflicts in the keymap, for example, several actions using the same shortcut. You can check that via Settings | Keymap | Edit keymap.
If it doesn't help, feel free to file an issue: https://youtrack.jetbrains.com/issues/FL