The bug

• If a Realm Owner does something that causes a player to be kicked (due to being uninvited, world option changes, etc.), it will always say Kicked by an operator. even though "Operators" (in Realms terms) nor the owner can use /kick even using the Configure panel (without uninviting and re-inviting the player, similar to running /ban and then /pardon on the same player in a short time frame).

Reason for the bug

• When the player does an action, the Realms program runs /kick <playername> for each player (since /kick @a isn't a possibility) on the console (or some custom external java function called by the Realms 'system' that 'runs' the realm), but does not specify a reason.

What I expected

• When a player was kicked off of a realm, it would show why (ex. "You have been removed from the Realm whitelist.").

What happened

• No matter how you try to kick a player off (more on that below), it will say "Kicked by an operator."

Reproduction

• You will need 2 players, one of which must own a Realm. I will refer to them as Owner and Observer (not to be confused with the block).
• Close and open the Realm while the Observer is online. The Observer sees a kick screen that says "Kicked by an operator".
• Change a setting in the Realm config menu (ex. Enable Command Blocks, Difficulty). Once again, the Observer will see a kick message saying "Kicked by an operator".
• Switch worlds. Unsurprisingly, the Observer gets kicked with the message "Kicked by an operator".
• Remove the Observer from the invited players list. "Kicked by an operator". What else would you expect?

A fix

• Add kick messages which are localized, ex. disconnect.realms.setting=Realm settings have been changed that require the Realm to restart.
• This would require a little tinkering with the server source code (since as of 1.12.1, all kick messages can only be entered as plain text) to allow usage of external JSON kick messages (which would also be useful for regular server administration).

Sidenote

This seems a little like a suggestion, but it is also a bug because neither Operators nor Owners can kick players (all players are granted OP level 2 which only includes singleplayer commands (not /ban, /kick, etc, and the Owner has extra permissions through the Configure GUI which cannot kick players without uninviting/reinviting them), thus the message is false.

Edits

• Changed a few things primarily for clarity. Also added edit and reason for bug sections.