It's called "internal consistency" and it's important:
https://rachellemnshaw.com/blog/2013/02/why-good-writing-matters-internal-consistency
https://lgestrella.wordpress.com/2015/01/21/the-importance-of-internal-consistency/
Arthur Conan Doyle said it best:
“It has always seemed to me that so long as you produce your dramatic effect, accuracy of detail matters little. I have never striven for it and I have made some bad mistakes in consequence. What matter if I hold my readers?”