Pokémon HOME has posted a Maintenance Notice in-app which has announced they will be fixing issues including a bug if people completed the Pokédex before February 14th, 2020 and received the glitched Magearna.

Pokémon HOME - Maintenance Notice - March 6th, 2020

Thank you for using Pokémon HOME. This message is to inform you that system maintenance has been scheduled for the following time period. During maintenance, all features of Pokémon HOME will become unavailable.

Maintenance period: 01:30 to 08:00 UTC, 03/06/2020 (scheduled)

Fixes:
A fix for the issue with the gift presented to players who had completed their National Pokédexes before 11:00 a.m. UTC on Friday, February 14, 2020.

There were issues with the previous gift that was distributed, and so the correct gift will be sent to eligible players' Gift Boxes after maintenance concludes. Those who already received an incorrect gift should also see some changes made to their data to fix the situation.

There was also an issue wherein the system failed to correctly recognize whether a Pokédex was complete or not, and players could not claim their gift despite their Pokédex appearing complete. In a future update, we plan to correct the conditions for completing your Pokédex and make it possible for those players to receive their gift.

We apologize for any inconvenience this has caused to all of our users.