Not sure if it's just me but I searched for all active games of the system Dungeons and dragons 5e and the first page worked fine but the second page showed for a search on dungeons only. I took a look at the string in the url and it went from the first page of:

link to "&gm="

to a second page of:

/?gs=Dungeons%20&%20Dragon...;af_sowner=1&p=1

I manually edited it to the below and it showed the actual second page:

/?gs=Dungeons+%26+Dragons+5e&a...;af_sowner=1&p=1

I'm getting the same thing looking for Dungeons and Dragons 3.5

First page:
<code>http://rpol.net/?gs=Dungeons%20&%20Dragons%203.5&sa=1&match=all&sort=alphabetical&search=search&af_general=1&af_mature=1&af_sowner=1&p=1</code>

Second page:
<code>http://rpol.net/?gs=Dungeons%20&sa=1&match=all&sort=alphabetical&search=search&af_general=1&af_mature=1&af_sowner=1&p=2</code>

You're right, the first page searched on the full criteria whereas the second only has the Dungeons part:

<code>?gs=Dungeons%20&%20Dragons%203.5 vs ?gs=Dungeons%20</code>

Assumedly a tiny bug in the code that's truncating the search string - one for the illustrious and all wise jase to take a look at.