Hi,
I’m new in using Mikrotik and kindly excuse my witlessness.
I got this issue regarding IOS users getting kick-off the network after sometime the lockscreen is enabled. 
I don’t have this issue on android users and my routerOs version is 6.26.
This causes too much annoyance to our users as they are asked again to log-in in our hotspot page. 
Is this a known issue? Is there any work around? I would appreciate any help.
you should enable cookie in hotspot
I did prior this post but it seems they are still being logged out, and it seems the cookie doesn’t work to them. But in android, everything works fine.
Is the phone is Low Power mode? Does the browser have Private Mode turned on?
I don’t think it’s really possible to help you here. I can tell you that my hotspots work with iOS correctly. and I can tell you that if iOS is in low power mode, or the battery is getting low, it will release the dhcp lease and turn off the radio when it goes to sleep. But without actually interacting with the hotspot and the devices in question, I don’t know how much anyone could help.
Personally, I don’t use public WiFi on my phone, so I don’t know what it does when it locks. I would suspect that it should disconnect, as being connected to an open network is never a good idea.
I will give it a try later and see what is really happening.