I have not seen this yet, so we’ll need additional context. Can you please use the ‘-src’ flag to show which data source is producing the name? Also, can you share the command-line arguments being used? Thank you

Spyse is the source I see.

[Spyse]           s2.x.net
[Spyse]           server.x.net
[Spyse]           c-domain__target--s1.x.net
[Spyse]           c-domain__target--m.x.net
[Spyse]           c-domain__target--www.x.net
[Spyse]           c-domain__target--server.x.net
[Spyse]           c-domain__target--ads.x.net
[Spyse]           c-domain__target--s2.x.net

My command line:
amass enum -src -d $domains -o $output

It's a nuisance, but not a problem. I just run a post-processor like so:
sed -i 's/c-domain__target--//' $output && cat $output | sort -u > $output

Thank you, @geeknik!

Took a quick look and.... Spyse completely rewrote their site, again. Now they are using IDs in the html that parse as valid domains, so the scraping regex will need to be adjusted.