I entered yesterday and explained to the agent I was entering on a spouse visa and showed my vignette and was also just directed through the egates and received no stamp.
This is an excerpt from the gov.uk website showing updated May 20 2019:
“You should see a Border Force officer and get your passport stamped if you’re coming to the UK:
for short term study of up to 6 months
with a Tier 5 Creative and Sporting Certificate of Sponsorship for short-term assignments (up to 3 months)
to carry out Permitted Paid Engagements
as the family member of an EEA national seeking to join them permanently in the UK and you’re a national of Australia, Canada, Japan, New Zealand, Singapore, South Korea or the USA
If you use the eGates, please see a Border Force officer before you leave the port to receive a stamp in your passport.”
Hopefully the stamp is no longer needed and this isn’t just teething problems for the new system. Their website seems to say so!