Since December 11 2018, Connect has been showing the previous day's weather for my activities.
This is probably a location-specific problem. It coincided with a change in the reported source for the weather, from "KALB" to "72518", which are the US and international designations for the same weather station in Albany, NY.
Displaying older activities gets the correct day's weather with the source reported as KALB. Only activities recorded on or after Dec 11 2018 get the previous day's weather with a reported source of 72518.
Note: not all browsers report the weather source - in fact, I think most don't. However, Firefox running on Linux does when you click on the weather widget to expand it.