"Adaptable" key bindings changes existing keybinding

[vigilancer]

Adaptive thing should be working only if binding was not set. But it ignores existing key if exist and rebind 'last-window'.

How to reproduce:
add to tmux:
...
unbind C-b
set -g prefix C-z
bind C-z send-prefix
...
unbind z
bind-key z resize-pane -Z
...
set -g @plugin 'tmux-plugins/tmux-sensible'
...
run '~/.tmux/plugins/tpm/tpm'
...

How to fix (workaround):
place
unbind z
bind-key z resize-pane -Z
after
run '~/.tmux/plugins/tpm/tpm'

What should be going wrong:
Plugin do not correctly recognize binded key.
Problem seems to be inside of 'key_binding_not_set()', in regex in particular.

btw, using OpenBSD's integrated tmux (it's 2.x I believe).