A picture tells a thousand words...
Before tamping...
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After tamping, showing that the grounds are below the razor.
View attachment 216880
After infusion, grounds have expanded and contacted the shower head.
View attachment 216881
...I know, I didn't clean the edges very well.
I have a new theory...
The puck might be more likely to get stuck if I don't remover the portafilter straight away. I wonder if things are cooling down and contracting behind the shower head, generating suction.