This is expected.
The point of
'paste' is to "avoid unexpected affects" when pasting text and insert-mode mappings are first in the list of features disabled by it.
'paste' option is switched on (also when it was already on):
- mapping in Insert mode and Command-line mode is disabled
- abbreviations are disabled
It also disables other options that cause Vim to modify pasted text, such as options related to indentation and text wrapping.
The only exception of keystroke that is special in paste mode (other than ESC, to leave insert mode) is a keystroke set by
'pastetoggle', if one is set. That keystroke will disable paste mode (when enabled), so you can go back to normal insert mode.
Also note that
'paste' mode is not really supposed to be left on, in most cases. Since you're disabling a lot of Vim features when you enable it, you typically only want it one for the duration of a paste and disable it right afterwards.
If you're willing to look at a plug-in recommendation that has useful keybindings for paste mode, I can suggest tpope's vim-unimpaired, which exposes these three mappings (from normal mode):
[op: Paste before the current line
]op: Paste after the current line
yop: Paste replacing the current line
In all cases, Vim will be put into Insert-mode with
'paste' on, and
'paste' will be turned off as soon as you leave Insert-mode with an ESC. The idea is that pasting is the only thing you'll do with that option turned on, so the sequence of one of those keystrokes, followed by the actual pasted contents, followed by an ESC will become a coherent operation.
:help pasting for that plug-in for more details.