Site Bug: Spoiler Code (?)

Short description:
When I edit my thread and click "save", spoiler-related code is automatically being generated and injected. Only through SEVERAL trials (and errors) was I able to get a working format. I still am unable to put a list inside; just getting the left alignment to work was difficult enough.

Thread in question:
[PS4] Snowbear's OW Boosting [$4k+ Traded/4 YEAR Member][Bitcoin ACCEPTED!]

Code that I TYPE:

Code:

[SPOILER="Additional Information"]
Duo Queue orders are available:
[/CENTER]
[LEFT][B]Win or Lose:[/B] Pay per match, the outcome of the game does not matter.
[B]Net Win:[/B] Pay for X wins, we will continue playing with you until we have won X more games than we have lost.
[B]SR Gain:[/B] Pay to reach X SR, we will continue playing with you until you reach that SR.
[/LEFT]
[CENTER][B]Leveling[/B]: FLAT rate of $3 per level (discounts on bulk orders!)

[/SPOILER]

After clicking save, realizing the thread does not look like it should, and clicking Edit again:

Code:

[SPOILER="Additional Information"]
Duo Queue orders are available:
[/SPOILER][/CENTER]
[SPOILER="Additional Information"]
[LEFT][B]Win or Lose:[/B] Pay per match, the outcome of the game does not matter.
[B]Net Win:[/B] Pay for X wins, we will continue playing with you until we have won X more games than we have lost.
[B]SR Gain:[/B] Pay to reach X SR, we will continue playing with you until you reach that SR.[/LEFT]
[CENTER]
[/CENTER]
[/SPOILER]

I can't even hazard a guess as to why this is happening. At first, it seemed to be "triggered" by certain edits, such as placing a colon ( ":" ) after the word "available:"
Next, I realized it occurred after playing with formatting, such as trying to left align the three sentences or trying to place the three sentences into a list.

However, on one of my final tries, I had the thread looking the way I wanted it to, clicked edit in order to copy and paste some code (in order to create this thread) and when I clicked save, the thread was back to looking strange! I literally clicked edit --> save without changing ANYTHING and the spoiler code injected itself.

Thoughts?