weird blue line under texts (just while typing, not dictation!)

Hey guys, I don't know if y'all have the same problem as me but I do find it annoying when there is a weird blue line underneath my text (as below picture) every time I start typing. Despite turning off dictation function, autocorrect function but it does not change anything. The line will disappear when I click space or other keys to finish a my typing which is quite inconvenient for me.

It is much appreciated to receive some explanation and solutions from y'all. Thank you and have a good day!