So I have this issue now with multiple units.
The problem is only in LUCI and I guess it's because both switches have interfaces labeled 1~5 instead of what's written on the box. It looks like it's working normally, but when you submit changes it mixes up the configuration for both switches. Especially adding a VLAN to one switch will make it appear on the other and doesn't save the VID, instead just saves it as 3 sequentially. This can be fixed in the /etc/config/network file, but is garbled again when submitting on the Switch page in LUCI.