Is it harmful to anchor your feet behind the bench while doing cable seated crunches?

I’m new to cable seated crunches, and a dilemma I’ve been facing is that I need to anchor my feet behind the bench to avoid my feet being lifted off the ground. If I anchor my feet I can do ~65 ibs but otherwise I can’t go much further than 40 without totally losing touch with the ground and still my feet will be slightly lifted during the beginning of the pull. Is it ok to lock my feet or it’s due to weak core and I need to keep my sole on the ground?

I’m 90kgs for reference.