The purpose of this report is to find out whether this behaviour is intended.
The slot IDs used for /replaceitem use numbers internally and for the feedback. The number used for container.0 and hotbar.0 is both 0. Usually slot names map to unique slot numbers, for example villager inventories use the numbers 300 to 307.
Generalised the post from "hotbar.0 works for item frames" to "hotbar.0 is the same as container.0".