USERNOTICE rendering some messages incorrectly

[zneix]

It seems to prepend value of system-msg to the message in frontend in this example:

2019-12

raw logs (with s/;/\r/g)

@badge-info=
badges=twitchconEU2019/1
color=#FF0000
display-name=MADMONQ
emotes=
flags=
id=f09acb77-b2d5-4608-ad3e-4c9678904312
login=madmonq
mod=0
msg-id=rewardgift
msg-param-domain=megacommerce_2019
msg-param-selected-count=5
msg-param-total-reward-count=5
msg-param-trigger-amount=1
msg-param-trigger-type=SUBSCRIPTION
room-id=22484632
subscriber=1
system-msg=reward
tmi-sent-ts=1575571146023
user-id=120654467
user-type= :tmi.twitch.tv USERNOTICE #forsen :A Sub shared Rewards to 5 others in Chat!