Some Typescript test files fail to execute with Mocha and ts-node - "unknown option"

I've been testing my prject for some time with mocha and ts-node with no issues. lately some of the test files started failing on execution returning the following:

error: unknown option '--require'

same result with -r

i'm using the same execution format for all files:

node_modules/mocha/bin/mocha --require ts-node/register ***path to test file***

some files execute correctly and some return the said error.

any thoughts?