How to reproduce:
1) Put item in hand
2) Run command:
data modify entity @s SelectedItem.tag.display.Lore prepend value "{\"text\":\"test\"}"
After running the command, the lore of the held item is changed, but the still game writes the error message: "Unable to modify player data" in chat. But this only occurs if the item already has a lore.
Linked issues
is duplicated by
Comments

Still present in 1.14.1 and 1.14.2-pre2
Confirmed. Only works if
tag
is already present on the item