Text filtering: behavior of current message is affected by behavior of past message from same origin

If there is this situation:

1. A text message is sent from a sender and gets classified as junk (by a text filtering extension) with the result that it gets send to the spam folder as expected.
2. A text message with different content is sent from the same sender and gets classified as allowed, however it also gets sent to the spam folder.

If the above is repeated but after step 1 the message is deleted, then in step 2 the message doesn't get sent to the spam folder.

So the presence of the message from step 1 being in the spam folder is having an effect on the behavior of step 2.

Expected beahavour (if so, why?), or a defect?