I have to imagine this is intentional, the damage reduction effect fades as soon as the bubble breaks, but the regen specifically has a timer and does not fade. If the regen did fade as soon as people step out of the bubble, then CU would still be relatively useless, as no amount of regen potency in it would make up for the loss of GDCs while using it.
It's a little unwieldy to use as it is anyway, because the regen effect is tied to being inside the bubble, you'll usually have to sacrifice a GCD or two to keep it up long enough for the server tick to apply it in the first place.
