mysql db user frequently get access denied issue while connection through proxysql

Here is my environment :
Azure ubuntu 18.4 instance with three mysql 8.018 version of mtaster slave
xxx.xxx.xxx.73 (mysql slave)
xxx.xxx.xxx.75 (mysql master )
xxx.xxx.xxx.79 (mysql slave)
Proxy sql version is
proxysql --version ProxySQL version 2.0.6-73-gc746bf7f, codename Truls
mysql client is mysql Ver 8.0.19 for Linux on x86_64 (MySQL Community Server - GPL).
I also change password authentication value in all mysql node config parameter to

collation-server = utf8mb4_general_ci
default-authentication-plugin = mysql_native_password

Still I get access issue from proxy sql connecting to DB server node for same user id and
password sometime it get access to mysql some time it giving access denied .

mysql -h 10.5.0.80 -u <mysql_user > -p -P 6033 -e "SELECT @@hostname ; ".

this script some time gets connected and sometime it won’t access and getting message access denied ERROR 1045 (28000): ProxySQL Error: Access denied for user ‘app_user’@‘10.5.0.80’ (using password: YES) "`I am breaking my head for last three days … need help urgently

Hi @debashis_mallick
 access denied ERROR 1045 (28000): ProxySQL Error: Access denied for user ‘app_user’@‘10.5.0.80’ (using password: YES):
1)  Verify user is present and user creation was done using proper mysql commands, and ensure at the end ‘flush privileges;’ was executed.

2) Also check for typo and use quotes for password.
mysql -h ‘xxx’ -u ‘yyy’ -p zzz -P 6033 -e “select xxxx;”