Training plan session not showing as complete

I am following a training plan and a session (Purple Unicorn) is showing as my having done it in Companion but not shown as completed and also not shown as completed in the Zwift app itself. This is since the latest upgrade although I didn’t have this problem the first time I used it after that. I have marked it now as completed outside as this was a fix suggested when this happened earlier in the year. Anyone know what the problem might be?