Mairsil, the Pretender and Quicksilver Elemental interaction

For some reason Mairsil, the Pretender doesn't get quicksilver elemental's ability even though quicksilver elemental is exiled with a cage counter on it. I was about to go infinite when I found out that mairsil didn't get quicksilver elemental's ability.

Here's the relevant ruling by Eli Shiffrin (Current WotC rules manager): https://twitter.com/elishffrn/status/897894399329222656?lang=en

Start by reproducing the Mairsil, the Pretender and Quicksilver Elemental interaction described in the issue, then read the linked ruling by Eli Shiffrin. The fix is complete when Mairsil correctly gains Quicksilver Elemental's ability while it is exiled with a cage counter.

Written by the indexing model from the issue text.