Asterisk EAGI audio while running AMD or other asterisk app via "EXEC"

Question

Is it possible to use "AMD" to detect silence in EAGI script and receive the audio on fd 3 at the same time?

Is this scenario supported or I am doing something wrong?

Simple demonstration bash script, which is run as EAGI(/home/agi/eagi.sh) from asterisk:

#!/bin/bash

log=/tmp/eagi.log
# Read all variables sent by Asterisk to array
declare -a array
while read -e ARG && [ "$ARG" ] ; do
        array=(` echo $ARG | sed -e 's/://'`)
        export ${array[0]}=${array[1]}
        echo $ARG | sed -e 's/://' >>$log
done

/usr/bin/dd if=/dev/fd/3 of=/tmp/eagi.tmp.out &>>$log &

### or just sleep 10 ###
sleep 1
echo "EXEC AMD"
read line # blocks until silence is detected by AMD
echo $line >>$log
sleep 1
### ###

kill -USR1 %1; sleep 0.1; kill %1
ls -lh /tmp/eagi.tmp.out >>$log

echo "EXEC HANGUP "
read line
echo $line >>$log

exit

What it does is it starts capturing the audio data from fd 3 via dd started as background process. When I have just sleep 10 instead of the echo EXEC AMD, after the 10 seconds, dd has recorded the full audio file.

However with "AMD", dd stops receiving data on fd 3 as soon as the "AMD" is executed (confirmed also via strace) and continues after "AMD" finishes. So while "AMD" is running, no audio is recorded.

Output in the logfile looks like this:

Working (with just sleep):

1522+501 records in
1897+0 records out
971264 bytes (971 kB, 948 KiB) copied, 10.0023 s, 97.1 kB/s
-rw-r--r-- 1 asterisk asterisk 958K Sep 24 10:16 /tmp/eagi.tmp.out

Non-working (with "AMD" which detected silence after 6 seconds, and dd was running the whole time but only 1 second before and 1 second after "AMD" was recorded into the file):

322+101 records in
397+0 records out
203264 bytes (203 kB, 198 KiB) copied, 8.06516 s, 25.2 kB/s
-rw-r--r-- 1 asterisk asterisk 208K Sep 24 10:13 /tmp/eagi.tmp.out

So is this some kind of bug in Asterisk, or just unsupported usage? I didn't find much info about EAGI in the Asterisk documentation, so not sure what is supported and what not. Version of Asterisk is 16.2.1 on Debian 10, the testing call was done via webphone on Chrome browser, audio passed via fd 3 was 48 kHz, 16bit, mono (maybe with some other audio format/codec, both fd 3 and "AMD" would work at the same time?)

EDIT2: Removed info about my complicated setup and added simple reproducible example.

EDIT3: During further debugging I used "EXEC Background" to output some short audio file to the caller and also during this no audio was recorded. So the issue seems to be not only with "EXEC AMD", but also "EXEC Background" and probably also other asterisk applications invoked by "EXEC".