I don't believe this is a bug.
This happens with a number of champions that empower their auto-attack (AA) with an ability that causes there next attack to have bonus effects. I am not a developer, but if the spell empowering the AA is (1) deliberately intended to reset the cool down of the AA, or (2) the spell is intended to immediately allow an AA to follow regardless of attack speed and when the last attack was landed, NOT when the animation takes place, and is activated before the target flashes, then it is logical in combat for the target to take the hit.
Vi is another good example: the target can flash away from vi during the animation of her "E" (devastating blows) and "somehow" sees the damage applied after they have flashed. The spell is activated and the auto attack is attributed to contacting the target. When you flash away from the AA you see the animation conclude after you flashed, but the spell was already activated before the flash. I think it is up to the developers to decide if there is some kind of balance to allowing mele-range champions this advantage of "spell locking" empowered auto-attacks. In a lot of ways I think it does. One thing we can know for sure is that, technically, the spell of the attacker empowering their next AA gets off before the flash, so the "now-ranged-attack" as you say has nothing to do with their AA range and everything to do with the spell applied to their AA.
I am now going to digress a little, but Animations do not, and cannot, always be directly linked to the true outcome of combat. Vi vs Lee Sin is another example. What happens when an unstoppable force meets an immovable object? When Vi ults (lets say the enemy ADC), but Lee Sin also ults Vi in the process, Vi gets kicked across the map --never reaching the ADC-- yet "somehow" the adc still is knocked up by Vi's ult and takes the damage. It has to happen according to the champion ability.
If Fizz's ability empowering his AA is activated just before you flash, I could see you taking damage from his next AA a plausible outcome. I think we need developer feedback to inform us of the intention behind Fizz's "E" (Seastone Trident).