The theory is you can’t overcook it - the biggest issue with Half Monty is not being able to hang on long enough to get the HR up high enough to make the FTP section meaningful quite often (i.e. the VO2 side of things kills people off fast (it does me) and the HR target is then too low in the FTP section, resulting in an FTP result that doesn’t really work.
You’re the opposite, so that’s really interesting - and as you’ve been smashing it at a good cadence on both sections, I’d say this is quite unusual.
It feels like its ‘one of those days’ where one big effort has simply crucified you on that day,and there’s not enough left in you for the next bit - which could in theory (total guesswork) be caused by residual fatigue, which is often characterised by HR struggling (but that usually then has a side effect of lower cadence as well).
The problem with the ‘internet’ here is this is all individual, so in the spirit intended - is there any chance that your FTP is higher? Forgive this comment (without history) as you are just as likely a professional athlete and know your numbers inside out. just a thought. If new to the structured training thing, it can sometimes take a few tests and similar efforts to get to that eventual number.
I just did one of these myself on Friday, one of the few successful HM’s I’ve done in the last couple of years, so happy to compare notes/data if it’s of any help.
EDIT: Dammit - this is probably irrelevant now - Mac has answered above. Duh. Should really read the whole thread. Last comment still applies though if you want