"-M" option does not work as expected with switch statement

[Hyunho Cho]

GNU Awk 5.1.0, API: 3.0 (GNU MPFR 4.0.2, GNU MP 6.2.0)
Operating System: Ubuntu 20.04 LTS
Kernel: Linux 5.4.0-39-generic
Architecture: x86-64

###     Without "-M" option work as expected

bash$ awk '
{
    switch ($1) { case "A" : arr[1] = $1 }
    print "value: " arr[1]
}
'
A                       <------------ this is a user input
value: A            <------------ the result as expected

### If i add "-M" option then value become "0"

bash$ awk -M '
{
    switch ($1) { case "A" : arr[1] = $1 }
    print "value: " arr[1]
}
'
A
value: 0           <--------- value become "0"

### No such error occurred without switch statement 

bash$ awk -M '
{
    arr[1] = $1  
    print "value: " arr[1]
}
'
A
value: A