Item being replaced

I had 64 warden spawn eggs in my first slot, nothing else in my inventory, the items had no NBT, then I picked up a sculk catalyst and it overwrote the warden spawn eggs. The catalysts had no NBT either. Don't have any proof as I wasn't recording or anything (im making a resource pack and if i record my game will lag a bunch if i open a recording software cuz my pc is shit)