My Octavia mk 3 (2019)’s daytime running lights get dim on switching on the headlights as well as the fog lights. 
I tried this out on a friend’s 2018 Octavia and it didn’t happen on his. 
Is this a normal occurrence with my 2019 model?

All DRL's should dim when headlights are switched on - clue is in the name, they are daytime running lights and are simply too bright for night use at full power.

I think they drop by about a third once headlights come on.

Yes that’s exactly what happens. Thanks chief.