I 100% agree the Concentration issue is a problem, but how exactly is the damage you describe different from casting Shatter? To me it seems like a semantics argument; as in, if they had thrown the bottle just next to you instead of targeted on you that would have been fine? Or is it the automatic damage from fire? Because that's a feature in PnP...
Shatter deals damage once.
Magic missile and heat metal deal damage multiple times, but they have big drawbacks in that MM is completely blocked by shield and heat metal requires concentration.
Fire arrows, fire flasks, and surface-creating spell can deal damage twice. This makes it so much more likely that concentration will be broken.
If you want a surface created by a thrown flask to deal guaranteed fire damage, then said flask shouldn't
also deal damage from hitting a target. You should have to choose between setting the ground or the single enemy target on fire.
See the following example for 5e oil, where you have to make such a choice.
Oil (flask): Make a ranged attack against a target creature or object, treating the oil as an improvised weapon. On a hit, the target is covered in oil. If the target takes any fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil.
OR
You can also pour a flask of oil on the ground to cover a 5-foot-square area, provided that the surface is level. If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area.
