RIOT HAS ANSWERED US:
"Wingfield (NA) - 2 months ago
Just want to help clarify some things about key drop rates:
Key drop rates are NOT linear and only wins have a chance of dropping a key. So you will earn key fragments much faster in your earlier wins than you will in your later wins. Or said another way, each incremental key fragment drop is harder than the last (on average... there is variance). This resets on a monthly basis for each player individually. The goal is to create a nice ebb and flow.
Grade does NOT impact key drop chance.
Premades DO increase drop chance.
Chests earned has no impact on key drop chance.
There is variance and randomness (with limits). It's unlikely that what happened to your friend will happen to you.
Hope that helps clarify :) "
This doesn't seem to be the case for everybody, but it seems that once some people get all 4 chests, they can not get any more key fragments from matches. I got all 4 chests on day 1 and haven't had another fragment since. I've just been stuck at 2. Win after win, nothing.
I've checked this with a couple people and we all seem to have gotten all 4 chests when it started happening.