default parameters in string substitution returning bad substitution

I have the following script:

#!/bin/bash

# usage: [path_to_script]/minecraft.bash [functions_to_run]
# example: /srv/minecraft/minecraft.bash stop "backup 10"

# -------------------------------- settings --------------------------------

# paths
BACKUP_DIRECTORY="backup"
SERVER_NAME="paper-296.jar" # the filename of your server jar
WORLD_NAME="matigcraft" # level-name in server.properties

# messages
BACKUP_KICK_MESSAGE="The server is making a backup, and will be back online in a few minutes"
BACKUP_WARNING_MESSAGE="The server will go offline to make a backup in X minute(s)"
STOP_KICK_MESSAGE="The server is shutting down for maintenance"
STOP_WARNING_MESSAGE="The server will go offline for maintenance in X minute(s)"

# amount of backups to keep when cleaning
BACKUP_AMOUNT=28

# JVM arguments to use when starting the server
JVM_ARGUMENTS="-Xms6G -Xmx6G -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4 -XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90 -XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32 -XX:+PerfDisableSharedMem -XX:MaxTenuringThreshold=1 -Dusing.aikars.flags=https://mcflags.emc.gs -Daikars.new.flags=true"

# -------------------------------- functions --------------------------------

# check if the server is running
function status {
  screen -ls | grep -q -w "$WORLD_NAME" \
  && echo $WORLD_NAME is running && return 0 \
  || echo $WORLD_NAME is not running && return 1
}

# if running, stop the server and wait
# call with number to set as warning delay
# call with additional text to use as warning message, defaults to STOP_WARNING_MESSAGE
# call with additional text to use as kick message, defaults to STOP_KICK_MESSAGE
function stop {
  status > /dev/null && {
    [ -z "$1" ] || {
      screen -S "$WORLD_NAME" -X stuff "say ${{2:-$STOP_WARNING_MESSAGE}//X/$1}\n"
      sleep $(($1 * 60))
    }
    screen -S "$WORLD_NAME" -X stuff "kick @a ${3:-$STOP_KICK_MESSAGE}\n""stop\n"
    while status > /dev/null; do sleep 1; done
  }
}

# make a backup of the world
# call with number to set as warning delay
# call with additional text to use as warning message, defaults to BACKUP_WARNING_MESSAGE
# call with additional text to use as kick message, defaults to BACKUP_KICK_MESSAGE
function backup {
  stop "$1" "${2:-$BACKUP_WARNING_MESSAGE}" "${3:-$BACKUP_KICK_MESSAGE}"
  mkdir -p "$BACKUP_DIRECTORY"
  zip -9 -r "$BACKUP_DIRECTORY"/"$WORLD_NAME"_"$(date +%Y_%m_%d_%H%M)".zip "$WORLD_NAME"
}

that gives me the "bad substitution" error on "say ${{2:-$STOP_WARNING_MESSAGE}//X/$1}\n" in the stop function when I call backup 10. ShellCheck thinks my syntax is fine, and I'm certain I'm running the script in Bash. When I do T="${2:-$STOP_WARNING_MESSAGE}" and ${T//X/$1}\n", everything works as it should.

I think this solution worked before without issues, but I could be wrong. Is there something wrong with my syntax? Or perhaps there is an issue with Bash after an update?