[MC-167027] "Error while deserializing" in console when converting a 1.14 world to 1.15

The bug

The following messages appear when a world is being upgraded from 1.14 to 1.15:

16:33:02.498
Error while deserializing minecraft:hay_pile
16:33:02.508
Error while deserializing minecraft:normal_tree
16:33:02.513
Error while deserializing minecraft:hay_pile
16:33:02.514
Error while deserializing minecraft:plain_flower
16:33:02.515
Error while deserializing minecraft:normal_tree
16:33:02.515
Error while deserializing minecraft:normal_tree
16:33:10.940
Error while deserializing minecraft:plain_flower
16:33:10.944
Error while deserializing minecraft:plain_flower
16:33:10.944
Error while deserializing minecraft:normal_tree
16:33:10.945
Error while deserializing minecraft:plain_flower
16:33:10.946
Error while deserializing minecraft:normal_tree
16:33:10.952
Error while deserializing minecraft:plain_flower
16:33:10.955
Error while deserializing minecraft:hay_pile
16:33:10.957
Error while deserializing minecraft:plain_flower
16:33:10.957
Error while deserializing minecraft:hay_pile
16:33:10.958
Error while deserializing minecraft:hay_pile

I am near a plains village, which is probably where these structures come from.