Strange bridge behavior

[sda3.14]

I am currently trying to setup a bridge between two VLANs.
I have created the two VLAN interfaces and enabled them, leaving all other settings on default.
I then created the bridge interface and added both VLAN interfaces. I assigned a static IPv4 and enabled the DHCP server.
This is where the weirdness starts:
The DHCP server does not work at all, and when I connect my client device and give it a static IP address instead, the firewall and the client device still can't ping each other. However, if I flush the firewalls ARP table while the client is already connected, it works until I try to reconnect the client.
Interestingly, if I remove the second VLAN interface (in which there is nothing but the firewall) from the bridge, it starts working flawlessly - including the DHCP server.
I have also noticed that when having both VLAN interfaces in the bridge, the ARP table includes two identical entries for the client device - one on the bridge interface and on the VLAN interface it is connected on (I am unsure whether or not this is intended behavior). This is the case both before and after flushing the ARP table.
Any help would be appreciated!

[Maurice]

Tunables are set?

https://docs.opnsense.org/manual/how-tos/lan_bridge.html#step-six

[sda3.14]

They weren't, but I just set them and that didn't fix it either.
On another note, I just rebooted the system, and now the behavior is even weirder:
The ARP table flushing trick doesn't work anymore. Instead, it just doesn't work at all - until at some random point it just decides to start working again (and after reconnecting the device, it stops working again).
Also, the ARP table entry on the VLAN interface is now gone.