I’ve been working on getting Forest of Thorns up to 7 stars. Typically when I leveled a troop from that kingdom, I would get 10 points toward power every time I bumped a troop up a level. When I got within 15-20 points of hitting 7 stars, I only started getting 2 points per level.
Is this something that was changed recently? I’m hoping it’s a bug, because otherwise it’s going to be incredibly painful to level up kingdoms past 7 stars (once that option opens up.)
Your 11th troop, 201st level and 31st trait (and beyond) give only 20% points. This is by design.
Yup, you get max points only for fully lvling/traiting 10 troops-worth of troops, so :
so 3250 possible points from 10 troops (but that’s assuming you trait / ascend and lvl to max all of them),
it also works as a “CAP” for those full points (as in you can spread those 2000 points from lvls and 750 from traits amongst more than 10 troops (i.e. having 12 troops at lvl 16 = 192 , lets say two of them are ascended and lvl’d to 20, that’s +8, and that adds up to 2000 points, any additional lvl on any of the troops will be above the cap already, thus diminishing returns kick in)).
like @Kharybdys said, anything above the cap gives only 20% of the points, so each lvl is worth 2 pts, each trait 5 pts and owning 11th (and every next) troops 10 pts.
Personally I’d suggest focusing on the traits a bit more, when going for 7 stars (I noticed that when 5-staring kingdoms I used mostly souls, so I’m relatively far away from the 30 traits “cap” in most kingdoms, and that’s 25 pts each - so 1 trait = 12,5 lvls worth 
when we consider being in the diminishing returns zone with lvls).
For more information on powering up to 7 stars (and eventually beyond), check out my guide: