Why does a 'pipe-to-awk' behave differently inside a function?

Question

I'm using alsa to fiddle around with the volume on a Bluetooth speaker I have connected. It's a 1-speaker setup; just something to provide some background.

I can 'get' the volume setting fm the CLI as follows:

$ amixer sget Master
Simple mixer control 'Master',0
  Capabilities: pvolume pswitch pswitch-joined
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 65536
  Mono:
  Front Left: Playback 22938 [35%] [on]
  Front Right: Playback 22938 [35%] [on]

I can 'set' the volume fm the CLI as follows:

$ amixer sset Master 50%
Simple mixer control 'Master',0
  Capabilities: pvolume pswitch pswitch-joined
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 65536
  Mono:
  Front Left: Playback 32768 [50%] [on]
  Front Right: Playback 32768 [50%] [on]

I don't need all this verbosity, so I decided to create a function, and two aliases in my ~/.bashrc file to reduce that. The function is not working as I expected:

From the CLI, this works fine:

$ amixer sget Master | awk -F"[][]" '/Right:/ { print $2 }'
35%

But when I put this into my function statement in ~/.bashrc, it works differently:

# functions & aliases for alsa mixer/volume control
function vol-get() {
   amixer sget Master | awk -F"[][]" '/Left:/ { print $2 }'
}
export -f vol-get

alias vol-up='amixer sset Master 5%+ > /dev/null && vol-get'
alias vol-dn='amixer sset Master 5%- > /dev/null && vol-get'

Re-reading ~/.bashrc & running the vol-get function yields the following:

$ . ~/.bashrc
$ vol-get
  Front Left: Playback 22938 [35%] [on]
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ?? what happened here ??
$ vol-up
  Front Left: Playback 26215 [40%] [on]
$ 

So - I'm close, but I don't understand why my | awk ... behaves differently inside the function than it does on the command line. Can someone explain and correct this?

---

Edits for answers to some of the Questions raised in the comments:

EDIT #1:

$ command -V vol-get
vol-get is a function
vol-get ()
{
    amixer sget Master | awk -F"[][]" '/Left:/ { print $2 }'
} 

$ awk --version
awk: not an option: --version
$ man awk
# reveals that I am actually calling `mawk`, Version 1.3.4

EDIT #2:

RE: non-printing characters; note this is a partial c&p from the output (i.e. verbatim):

$ cat -vet ~/.bashrc
...
function vol-get() {$
    amixer sget Master | awk -F'[][]' '/Left:/ { print $2 }'$
}$
...

Answer 1

This is a half answer, because I'm not sure of the root cause:

In the function definition, substituting

awk -F"[][]"

with

awk -F'[][]'

Fixes the OP's problem, reporting just the value 50%. I've been unable to reproduce this via my own WSL box as it works with both variations.

GNU Awk 5.0.1, API: 2.0
GNU bash, version 5.0.17(1)-release
Kernel: 5.10.102.1-microsoft-standard-WSL2

$ command -V test_awk
test_awk is a function
test_awk ()
{
    echo 'Front Left: Playback 32768 [50%] [on]' | awk -F"[][]" '/Left:/ { print $2  }
}

$ test_awk
50%

Which is expected, since according to the bash manual:

Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !.

No mention of [] there.