It’s hard to say really because there’s ‘head heaviness’ and also where the head feels too heavy to keep upright, ie to heavy for your neck muscles to manage. Both slightly different feelings or they are for me. Obviously when reclining much of the weight of the head is taken by the pillow, chair headrest or whatever. Literally reduced load.
The inability to do this is a strong indication of a balance disorder. I suffered with this for years. I went years unable to sit up at a dining table for that reason. Would have made Eating Out virtually impossible had it not already been so due to photophobia/visual vertigo but that’s another story. I changed all our easy chairs to ones with high backs and headrests and got rid of the sofa completely. I know other people who have done likewise. Visiting anybody else’s home was a nightmare. I still detest my favourite sister-in-laws sofa. For years my head had to have constant support.
No idea what a forward fold in yoga looks like but I suggest your tolerance of that is purely due to Habituation. One tends to retain trained responses despite the development of the condition.