Your Eevee was fighting an Alolan Rattata, right? If so, the reason is the Rattata had the ability Hustle. Hustle increases the bearer's Attack stat by 50% in exchange for decreasing the accuracy of all its physical moves by 20%. So in this case, Tackle had 80% accuracy and had a chance of missing.
The description that shows when an attack doesn't hit is not specific to either accuracy or evasion if I recall correctly, so while Eevee "avoided" the attack, it was Rattata's accuracy that allowed that to happen.