I do not know why Sprintax told you that you were a resident. If you were not in the US for 5 years or part years on an F1 visa, then you were a nonresident up until October 1, 2018.
Then starting October 1, 2018 you can start counting your US days of presence for the substantial presence test. You had less than 183 days of presence in the US in 2018. So you were a nonresident in 2018 and need to amend your return to file a nonresident return with Sprintax.
You are a US resident starting January 1, 2019. Please see example 1 https://www.irs.gov/individuals/international-taxpayers/alien-residency-examples
Unless I am missing something. you should file an amended return.
