Hello Euginken,
I tested three different scenarios with a t-wave using ferrothorn vs gyarados
1.
lead ferro | lead gyarados
twave | icefang
lum berry activates
2.
lead ferro | lead nincada
twave | switch to gyarados
lum berry activates
3.
lead ferro | lead gyarados
twave | dragondance
lum berry activates
as you can see, everytime the lum berry activates itself after the gyarados got paralyzed, so it was certainly just a bug with lumberry and not with the pokemon in specific