This is just a guess, I can’t give you an example, only where to start looking.
I’m pretty sure that this the actual behavior of the Date Time Picker, like you said.
I think that in order to do this you will need to alter the time zone somehow in the date/time picker, or the time picker as the case may be. Take a look at these hooks on the custom JS page https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/
Past this, it should be possible to alter the time used for the specific field, I think. You may need to “Localize” https://codex.wordpress.org/Function_Reference/wp_localize_script your custom JS file with information to use when initializing the field.
