VLAN tagging gone wrong?

We have an issue where no matter what switch we use, it randomly puts clients on the wrong VLAN, no matter what the vlan tagging on the port is.

If we disable DHCP on the secondary vlans, the main VLAN works.

If we enable DHCP it starts assigning addresses from the wrong VLAN.

When enabling DHCP on tagged VLANS, we experience random DHCP assignments on the tagged VLANS.

Example: Switch is configured with VLAN 1 and 40, the port is set to VLAN 1.

The firewall then assigns an IP from VLAN 40.

This seems to be an issue with the firewall, what do we do here?

Aruba switches with VLAN 1, 10, 20, 30, 40, 50, 60, 70, 80

4 Switches, all of them have the issue randomly.

Watchguard firewall that acts as DHCP server for all VLAN's except VLAN 20 that uses DHCP relay to Domain controller.