I just noticed this as well! Even older indoor activities where I used to be able to see the HR graph I cannot see them now. What gives!? What's weird is in the GC dashboard page, the activities tile *will* show the HR graph. It also shows up in the linked activity on Strava, just not on the GC full page view.
When I log an indoor cycling activity, my heart rate chart does not show up when I view the activity in Modern view; but I can see it in Classic view.