Well the reason is, that you seem to have missed that the string gadget for entering the search string accepts AmigaDOS wildcards. Therefore, the [ is a special reserved character and have to be escaped to match the literal character itself. So if you want to search for a [ character with an AmigaDOS pattern aware method you have to use:

'[SPAM']

where the ' character escapes the reserved state of the [ and ] chars. However, even more precisly would be to use:

'[SPAM']#?

instead and turn off the "Substring" option. That would then match only subjects that actually start with [SPAM] as there might even be non-SPAM messages contain that string somewhere.

NOTE on searching for [SPAM]:

Matching SPAM messages on the literal [SPAM] marker in a subject line is more or less a bad idea as it would cause (even if correctly escaped) many non-SPAM messages to get filtered incorrectly. Normally SPAM filter add an additional header line like 'X-Spam-Status:' to each checked email. So to reduce the amount of incorrectly matched mails you would be better off searching for that optional header line and search if it starts with "Yes".