Hard to say, might be the way the boot fits you or might just be that it's a really tough boot to break in. The digging in across your toes concerns me more than heel slippage, based on my experience.
Here are a couple of examples from my past:
Around 2017 I bought a pair of Crispi Idaho's, felt great on flat ground but I walked up the stairs a few times and they crunched the top of my foot bad. I didn't think that was going to go away so I returned them right away. Maybe it would have gone away, who knows, but I wasn't going to destroy my feet for months to find out.
In 2012 I bought a pair of Kenetrek Mountain Extremes, after 50 miles I still got heel blisters bad. Around maybe 100 miles things improved some, by 150+ miles they were the best boot I've ever owned. I still made sure to bring along tape and blister pads on every trip for two years just in case. That said, I still have them and absolutely love them. I had them re-soled two years ago and still going strong. I can hike 10-15 miles and my feet feel great at the end of the day.
None of that is meant to disparage or promote either company, I think it had more to do with the Crispi not fitting me right and the Kenetrek being an extremely stiff boot. So much of it is how a boot fits each person differently.