Unable to set LAG parent on interface which does not belong to VC master device

[candlerb]

NetBox version

v2.11.3

Python version

3.6

Steps to Reproduce

1. Create testsw1 and testsw2 (of any device type which has interfaces: I used Netgear GS724Tv4, which has 26 interfaces, g1..26)
2. Bulk rename g to g1/ on testsw1, and g to g2/ on testsw2
3. Create a virtual chassis name "testsw", members testsw1 and testsw2 (members selected in the virtual chassis creation page). This puts them in positions 1 and 2
4. Edit the virtual chassis and select testsw1 as "master"
5. Go to testsw1 and look at the interfaces tab. Notice that all 52 interfaces are shown: g1/1..26 and g2/1..26, as if this were one switch
6. Add interface, name lag1, type LAG
7. Starting from master device interfaces list: click pencil next to interface g1/1 to edit it (this goes to /dcim/interfaces/NNN/edit/?return_url=/dcim/devices/MMM/interfaces/). Set LAG to lag1
8. Using the same method: edit interface g2/1, try to set LAG to lag1

Expected Behavior

I should be able to set lag1 as the LAG for both these interfaces

Observed Behavior

Interface g2/1 does not allow any selection of LAG

I'm pretty sure this used to work with v2.11.2, but it doesn't with v2.11.3