In this passage:
Maxume vero sunt admirabiles motus earum quinque stellarum quae falso vocantur errantes; nihil enim errat quod in omni aeternitate conservat progressus et regressus reliquosque motus constantis et ratos.
Source: M. Tullius Cicero, de Natura Deorum 2.51. O. Plasberg, Ed.
why does Cicero write constantis rather than constantes? I expected constantes since it modifies the accusative plural reliquos motus.
This Wiktionary table suggests that many third-declension adjectives allow both -ēs or -īs for their accusative plural.
Why and when is this legitimate? And considering that the genitive is constantĭs, didn't writing constantis confuse Roman readers, especially if they didn't mark the vowel length in -īs?