Do I still have to file an amended return if Schedule K-1 forms were mailed after March 15, 2019?

Yes - The IRS requires you report income from all sources.

If you filed your return before you received the K-1, you need to amend your return to include it.  To amend your return, please follow the steps in this FAQ: 

https://ttlc.intuit.com/replies/7276601