this flag generally makes the spells to preserve their noted full casting times as they get stolen by a Rubick and used by him

But this isn't what this flag should make...

There are currently 4 abilities that have this flag, there should be more, I will make a seperate thread/report for the missing ones
Here are these 4 abilities, their current casting times on both their actual owner and Rubick, and how long their casting times should actually be on Rubick

Requiem of Souls from Nevermore - 1.67 secs - 1.1 secs - should be 0.57 secs less
Assassinate from Sniper - 2.0 secs - 1.8 secs - should be 0.2 secs less
Teleportation from Furion - will talk - will talk -
Poof from Meepo - 1.5 secs - 1.1 secs - should be 0.4 secs less

About Teleportation...
Just seen that it has 3 secs of casting time in DotA for the Furion himself already, which is incorrect and should rather be 3.33 secs, will have a seperate thread to itself
Also, Teleportation should act completely the same on both Rubick and Furion, so the DOTA_ABILITY_BEHAVIOR_NORMAL_WHEN_STOLEN works as it should for that specific ability

Someone can make a video with timer on it if required, I can't
but I can explain why

These abilities have some additional casting time set in their ability data. This extra duration gets added to the casting unit's own casting time. Nevermore for example has 0.67 secs of casting time to himself, with the +1 Requiem of Souls has 1.67 secs of casting time on him. On Rubick, it should be 0.1 + 1.0 = 1.1 secs

Some other abilities on the other hand, like the Blink which Teleportation uses, disregards the casting time of the casting unit altogether, so they turn out to be correct in Dota 2 when used with the DOTA_ABILITY_BEHAVIOR_NORMAL_WHEN_STOLEN, at least it would be if the Teleportation had the correct casting time to it