To my understanding supervised solo flight is only allowable to someone who does not hold a licence. If the pilot holds a licence and the rating is lapsed then it is training as required to pass a test. There is no room for solo practice as either the pilot is ready for test or requires further training, there is no in-between.
I had this issue with a local examiner who decided to report me for it and refuse to do a test until it got sorted out.
The CAA confirmed it was acceptable for lapsed SEP's class ratings as previously they had a requirement for solo time after a certain period. But this was 9 years ago now.
Now it could very well be that it has changed under EASA. But there are lots of instructors that have done it and will continue to do it.
And what's the safety case to stop it or not allow it?