First thing, while the [Number Plays] field is an integer value, its default value is blank. So you should really be testing for blank rather than zero. Blanks are handled in different ways in MC for different fields, so it would be good practice, although the [Number Plays] field seems to be handled as expected. i.e. Blank = Zero. The expression would be [Number Plays,0]=[], so that you are testing the raw data value. Read about why here:https://wiki.jriver.com/index.php/Expression_Language
I just tested using audio files rather than podcasts on my Workstation local Library and with it connected to my HTPC MC Server as a Client. Both worked the same way, and correctly. Both required a refresh of the Smartlist to have the played tracks removed from the Smartlist. Podcasts should work exactly the same as other audio tracks.
I'll just note here also that you said Playlist in your last post. I'm sure you meant Smartlist. Because of course a Playlist is static and isn't going to update once a Podcast is played. You said Smartlist in your first post, so I'll take it the second post was just an error.
For a MC Client to update the MC Server, it has to sync back to the server. It usually does this pretty quickly, but not instantly. Also, every time a MC Client is opened, it gets a fresh copy of the Library from the MC Server, overwriting the previous copy of the Server Library which is on the Client.
So if your habit is to listen to a Podcast on you HTPC MC Client, and then fairly quickly close MC, it won't have had time to sync back to the Server. So the new value of [Number Plays] isn't synced back to the Server, and then the Client copy of the Library with the new value is overwritten when it is next started, so the change is lost.
If you want to close the Client quickly, but want the sync to happen, run it manually by selecting the Library under Playing Now and clicking the "Sync Changes with Library Server" button.
You also need to have "Options >Media Network > Authentication" set up on the Server with a Username and Password, and the Client needs to (will be forced to) log into the MC Server so that Authentication and Synchronisation works.
So, does any of that help? If not, try the sequence below.
1. Check the Smartlist on your server. Note a podcast you are going to test with.
2. Start the Client.
3. Play the Podcast.
4. Refresh the Smartlist on the Client. Did the played Podcast disappear? It should have at this time.
5. Sync the Client to the Server manually.
6. Check the server Smartlist. Has the played Podcast disappeared? It should have at this time.
7. Close the Client.
8. Restart the Client.
9. Check the Client Smartlist. Has the played Podcast still disappeared? It should have at this time.
If the above sequence doesn't' work, provide all details of your Smartlist configuration. Screenshots are good.