It appears that the increase comes from events with “googleweblight” in their useragent string.
I haven’t found any fresh information about how Web Light handles cookies and whether it has changed its behaviour.
Have any of you guys noticed the same? How would you address this? Thinking about opting out of it following their instructions here: https://support.google.com/webmasters/answer/6211428
About 5% of our events are sent from Web Light users so it does have an impact.