Hi,
Stalker Portal and Playback limit with this protocol is always controlled by Provider. 3hours are set as standard in this protocol. Maybe some other boxes/ iptv apps ignore this setting, Formuler not but your Provider should know that this restriction exists.
XC Api Portal and M3u are not controlled through Provider and you can set by your self the Playback limit or turn off. In your case i would change all 6 subs from Stalker to XC Api to have full control and bypass the Playback Limit.
Any progress in what causing the portal mode to generate black screen?