Okay, that means they anyway end up in wrong VLAN since switch's default VLAN is 1. There are some notes in user guide stating "Ensure that the native VLAN of the IAP and uplink are not the same." That would solve my problem without configuring each IAP separately, but why is that a bad idea i.e. what breaks?
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Edit: how stupid of me. :) Even if I put the Native VLAN to 15, they would end up at VLAN 1 since that means they are being sent untagged. I would rather have to either configure each IAP with Uplink VLAN 15 or the switch port Untagged 15, right?