DNS not correct resolved intermittent on Android Oreo

Hello

need to get file from local store server - local store server given via DHCP via Primary DNS, Secondary DNS provided for backup if local store server not operating
system works for years with Windows CE (MC32)

now moving to TC51 (02-13-15.00-ON-U19-STD)
https://developer.android.com/reference/java/net/HttpURLConnection is used to request the file

sometimes not the Primary DNS is contacted by the device but the secondary DNS - although primary dns is available

sometimes can not really be defined :
- maybe after a restart of device it works ok again,
- or two restarts
- or there are devices which never show the issue
- or there are stores which have not this issue with all of their devices

also using Chrome Browser it will not resolve the primary dns correct

but when starting worryfree-wifi and do a ping to the DNS Name it will immediately return the correct IP Adress
and AFTER doing ping in worryfree wifi also it is correct resolved inside the app and with the Chrome Browser

in WLAN Info the order of DNS is always given correct.

any ideas are very much welcome !

thx, Christian