LTP and LTHRP how do they relate?

I use heart rate for my outdoor Z2 rides . I’ve noticed that when training indoors, where I have a power meter on my H3, sometimes when I do threshold workouts they don’t seem very challenging and my HR is in the Endurance HR zone. I’ve been doing TR for 3-4 months and I’ve yet to have one of those workouts where I feel like I’m suffering and spent. Except for my 20 ramp tests. VO2 max intervals feel the same(not very challenging) but I know I can’t rely on HR for short burst.

Could I just not be a good test taker? Should I try the 8 minute or the 20 minute test to see if there is a difference? Anyone see a vast difference in your FTP using the different methods? I’ve been riding forever and “racing” for about 10 years off and on. But I’m new to training with power. Before it was HR and or perceived exertion. I know theres is a lag in HR but I’m not talking about short bursts.