Update: I did some more research and found out that, after all, I’m not supposed to use date_default_timezone_set() with WordPress 5.3. As stated here https://make.wordpress.org/core/2019/09/23/date-time-improvements-wp-5-3/ :
“don’t change PHP time zone with date_default_timezone_set() (this one is hard requirement for correct core operation!)”
So I’m currently working on modifying my functions to work the proper way, by getting the ‘gmt_offset’ option and calculating it to process the current timestamp instead of changing the default timezone. That way ACF seems to work properly, too.
