FR24 has been very random for me the last year, I think it was when they started tightening their usage to encourage users to pay up for the full version (a not unreasonable wish). My map sets itself up at several different locations as and when it chooses and I've always put this down to where the main node for the system is, or thinks the user is. Sometimes it's just for a short while, other times several days.
A while back they started masking many private flights or switching off tracking when under a certain height to obsfucate the departure/arrival point. I wonder was this in response to legal action from some disgruntled flyer who wasn't aware their aircraft was eminently traceable by such internet apps ?
Btw, I've just checked today and FR24 has me centred on the Pennines in the UK, whereas yesterday it was southern Sweden.