Noticed this the other day when behind an Ioniq, I knew immediately it was because of regen braking. I love Technology Connections for taking it a layer deeper and going into the regulations and industry experts.
Sounds like an Ioniq specific oversight and bad design not a general problem with break lights
If you watch the vid, he starts out with the Ioniq's problem but then goes into discuss how their implementation actually meets the regs and therefore the regs are bad.