What caused a layer of film to form over the lemon curd?

Hi Emma, I'm a bit late to get onto the bandwagon but I'd like to get some troubleshooting help, please. A film formed over my lemon curd - would you have any idea what might have gone wrong? Baked too long? Too little? I measured out almost everything by weight , and only went volumetric for the lemon juice and zest. It was still slightly (barely!) jiggly in the centre when I took it out. Thank you in advance!