When I try to save the featured image in portfolio it isn’t saving. This obviously means that the image doesn’t appear on the portfolio page – http://datumpoint.org.uk/portfolio/
I hope you are well today and thank you for your question.
I could confirm the issue on my test site which occurs when using Block Editor therefore notified the theme developer about this so that it will be fixed.
The issue will be fixed ASAP.
In the meanwhile, you can resolve it using classic editor as described in the below pages.