I did one at Losinj LDLO in Sep 2019, unintentionally. Passports not checked once the police realised were were not entering the country (it was a fuel stop only).
No idea if this is officially normal. It may be for Croatia as far as they are concerned but the key point is that if you flew say Slovenia – Croatia – Italy (note: Croatia is not schengen) and just did a fuel stop in Croatia, then you absolutely would need to arrive at an immigration airport in Italy, with appropriate PNR etc, otherwise you are gonna get my Padova experience 
And this is the central issue with “tech stops”. It is the “next stop” which doesn’t give a damn where you departed from before arriving at the “tech stop”.
Pehu wrote:
I got the response from polish authority:Dear Pekka,
You don’t need any special permission or authorization form CAA PL. In your ATC Flight Plan in field 18 add RMK/EPSY fuel stop only.
Please be advised that some airports are working on reducing capacity and operation hours.Hope this clears issues for everyone else too.
Pekka
@Pehu, that’s a very good response and eminently pragmatic since the fuel station at Mazury is on the opposite side of the field from the terminal and associated facilities.
I’ve used Mazury as a fuel stop for a Polish national flight (so not relevant to this thread as such) and everything was completed at the fuel station. The airport operator sent a man over in a car to collect the landing fee while the refueler was topping me up. No need to cross the field (or runway) to the terminal to complete the admin. Taxi to fuel station, refuel and pay fees, taxi to runway and take off. All very efficient and highly recommended!
