It works both ways. In 2008 they didn't share in the losses in 2014 they don't share in the gains. it was assumed it all washed out in the end back when these kinds of plans were common. I would add back then daily plans were not cost effective for small plans so there wasn't much choice.
If this really causes an issue the solution is daily valued. What you are describing is a big reason why there was a push to daily value. People didn't like the fact people could end up not getting gains or losses on the months it took to finish the annual work.
But running an annual plan the way we are describing is legal and like I said back in the day common.
Good point GMK, unless it's applied uniformly to all employees it might violate the non-discrim regs if, as and when they're issued and effective but in the meantime should be OK