Hello!
I have the same issues with my fire TV 4K Stick. I tested a lot of apps like Perfect Player, GSE, Kodi and so on. I must say that the smart IPTV App is the most comfortable for me. Since it works perfect with my Fire TV Stick 2ndGen. And i use it for over three years.
But now it is a pity that i have that kind of issues with my Fire TV 4K Stick. I wrote with the support of the Smart IPTV app. He answered me that he can't solve the problem on his side. The "4k Stick" is the only device with Android 7.1.2 (which is the basis of Fire OS 6) which make that kind of problems. As far as i know, Fire TV with 4K Ultra HD (Fire TV 3rd Gen.) don't have that problems.
So maybe in the future Amazon can fix that problem.
I found out some things about the problem. I tried various iptv provider and mainly the same channels doing the same problems in various IPTV streaming apps which use the Hardware Codecs from the 4kStick. Kodi is working with most of the streams but isn't very comfortable (epg from different sources, no zapping of channels). Then i installed the mx player and made the "sw mode" for networkstreams standard via settings. When i switch to a channel who can't be loaded i push the enter/ok Button for some seconds an press the down button (on the ring) in the menu. Then i choose the MX Player als alternative player. So all of the streams are working in mxplayer software mode.
But finally it is not more than a workaround until amazon maybe fix that problem. Because if i want to return to the app after watched one of these "problematic" tv stations, the app freezes and finally crashes after some seconds. If i am zapping to a channel which isn't working, i force a crash by i pushing the Enter/OK Button until the menu pops up, press ok again and press ok for the 3rd time to restart the app. In my chase the Operation System freezes for around 5 seconds and the the app crashes. Then you landing on the Starting Page of your stick. After restarting the smart iptv app you have to switch the tvstation fast to another (working one) and the app is working again. So you don't need to reboot the 4k stick.
But this is the best worst chase solution. By the way, 2 days ago i installed the smart iptv update 1.7. But like support of the app said, the playback problem is still there and can not be fixed via app if the app needs the codecs which are provided by the stick.
So Amazon is hopefully will fix the problem soon.