If you think it's a conundrum now, aka. 'square peg/round hole' just wait for the real divergence to laws in the UK v. the EU to begin... we've not even started yet. Things are going to get very 'Frosty' and, sadly, turbulent. The threat of violence in NI is very real.
One of the issues I've always had with Brexit is the MSM assumption the entities of the UK and EU remain the same entities after Brexit, to keep thing simple, when discussing issues.
We have absolutely no say-so on what the EU does from now on, it can become anything it wants.