Hi Ihor,
The most likely reason in why you are seeing planes disappear and then reappearing from the map when they approach Donetsk is that there is no coverage in the region in where the plane flies through.
Plane Finder uses ADS-B technology to detect and show planes in a region on Plane Finder. Most of the planes flying through the area around Donetsk are ADS-B equipped and only show up on the Plane Finder website if there is an ADS-B receiver in the region as well. This ADS-B receiver picks up signals from the ADS-B equipment from an aircraft and this feeds on its information such as location, speed, altitude, callsign, heading and squawk into the servers at Plane Finder. This triggers the aircraft icon which appears on your device screen. However, when there is nobody available in the area to set up and share ADS-B data to Plane Finder, the plane simply cannot appear on Plane Finder. When planes approach Donetsk, they fly through an area with ADS-B coverage and it shows up on Plane Finder but when it flies through a 'Dead Spot', there is no information which can be sent to Plane Finder meaning that the plane is invisible to the website. When the plane finally passes through the 'Dead Spot' and once again flies through ADS-B territory, the communication is restored and the plane appears on Plane Finder. Aircraft can fly through many dead spots in one single flight and this is common for aircraft all over the world.
If this is the problem, then planes flying through Donetsk in real-time should also disappear from Plane Finder temporarily before reappearing after they have flown past the region.
Plane Finder is constantly searching for people to share data with the website and this helps increase the coverage as well as the helpfulness of the website.
I hope that this helps Ihor and please don't hesitate to ask if you have any other questions.
Shine Wang