How to freeze mango puree?
I usually freeze mango slices in a ziplock (note that it's best to place it in a single layer in the freezer)



then I let them thaw for a few minutes (3-4 minutes) before I blend them and that's my go-to mango smoothie/puree recipe. 

The issue with your method might be that you let it thaw completely for a long time which is not really needed. Another issue could be the freezer bag you used which probably is not airtight, that also could affect the taste.