The biggest concern is if its the heater core or hoses going to it .
so first (sorry the only way) dip finger in it, touch tongue ..if its sweet, then its coolant.
other possible causes are discussed here:
viewtopic.php?f=54&t=220622not as new as yours its a 2017..but no one else has come in complaining yet about a later one.
The link...sorry don't want to go through everything again when it was already done a couple of months back.
BTW - we went through everything - don't think it was resolved!
all the best..mark