Feel more tired at the same HR?

I’ve improved and can run the same or slightly faster 5k speed at a lower HR now than before

But when I try to push it and run at the same high HR as before at a higher speed, I feel a lot more tired and hit a kind of invisible barrier I didn’t have before at that HR. Before, I’d get hit with the high lactate symptoms and feel like my lungs and throat are burning but I could still keep going at the same speed at the high HR but now I feel a weird tired feeling and need to slow down

Note that these are runs at my 5k race pace, so close to VO2max. Is it normal to feel that it’s harder to stay at a higher HR after you get used to running the same speed at a lower HR?