This spell used to have a 100% hit chance. However, this is no longer the case. Now it is a ranged spell attack that uses your normal spell attack roll.
The way this cantrip works, is that the target of the cantrip has to make a DC 20 Saving throw that is not affected any bonuses or penalties. Most of the time the target will be hit by the normal 1d10Fire damage, but if the target succeeds the saving throw they will also be hit by a Fireball explosion.
This means that targeting something that can not roll for saving throws like the ground or an object, will always shoot a normal fire bolt.
If the target succeeds the saving throw the combat log will not only show a completely wrong calculation but it will also say that the target failed it.
Unlike normal Fire Bolt, this cantrip does not scale with your level and will always deal 1d10 damage.
cantrip, but has a 1/20 chance to cast 
instead for free.
Action
1d10 (1~10) 
Fire 
Ranged spell attack roll
Range: 18 m / 60 ft
Saving throw that is not affected any bonuses or penalties. Most of the time the target will be hit by the normal 1d10
Fire bolt on the Forgotten Realms Wiki