Update: Epic are looking into it, confirmed to be unintended:

The ‘Double Pump’ strategy was removed a few patches ago by adding a pump animation in between shots, but this seems to have changed with Patch v3.5.0.

In the patch notes it was stated that Epic had “Removed the reloading animation on the Pump Shotgun” but this should have been counteracted by a short equip time being added to weapons with a high damage output, including the Pump Shotgun.

It doesn’t appear the fix is working, however. As demonstrated in the clip below from Reddit, the ‘Double Pump’ strategy looks like it has made a comeback:

Double pump is back…? Why? from r/FortNiteBR

It is unclear whether this is intended or not right now, we’ll update this post if there is any official word from Epic regarding the matter.