Seems to be a different attitude towards the European elections than the local ones. Whether that is usually the case or if they're being more compliant in the arena of Brexit I'm not so sure.
I suspect the restrictions end up having a different effect because they are a proportional vote, party is more important than candidate.
The other big difference this time is that they weren't meant to be happening and the UK was only confirmed as definitely taking part last week so its probably caught some places off guard (even though the rules have applied in some areas for the recent local elections)
The rules are here by the way:
Interesting that if a programme has booked a guest before the election period, they are allowed to appear. Possibly explains last week's guest on HIGNFY
I think there is also the issue of 'balance across the piece'. If HIGNFY had a number of party leaders or similar senior politicians scheduled to appear during the EU Election Period, they would have achieved balance over that period. If Heidi Allen was the only party leader (or senior politician) booked for the shows during that time, it would be trickier.
However the argument about the EU elections potentially not happening is a pretty poor one. There was always a reasonably strong likelihood that EU elections were going to happen once it was clear the UK Parliament wouldn't support a 'No Deal' Brexit.
I do wonder if this is an Indie production company thinking they could sail close to the wind, and the BBC Editorial Policy department exerting some control.