DNS domain suffix on connection properties not retained

I added a DNS domain suffix to my TCP/IP connection [network connection->
Properties->General->Internet Protocol (TCP/IP)->Properties->General->
Advanced->DNS->Append these DNS suffixes (in order)->Add], and it has
remained in my DNS suffix append list for months.

Recently, I found that I also need another DNS suffix. However, this
second suffix keeps disappearing (while the first remains). I'm not
sure when it gets removed; perhaps each time I disconnect?

Can anyone suggest possible reasons why one suffix remains and the other
needs to be added repeatedly? What can I do to permanently add the
second suffix?

Thanks in advance for help.

Regards,
mike K

PS: I did not originally need the second suffix. At some point the DNS
stopped automagically translating http://name/ and began to require
http://name.domain/ where the domain is the second suffix mentioned
above. I'm not sure what happened there, either.