As expected, I do not see anything unexpected with the household parent config in that form.
I guess that still brings us back to some kind of race condition between launching the form and changing which contact is in context… I have tried to recreate similar behavior by slowing down the CPU/Network and clicking around in the app, but so far I have not been successful. (This is not surprising, though. If it was easy to produce this behavior, I guess it would be much more common.)
The main lingering doubt I have is that if it is a race condition, why has it only been observed in this deployment? With that prompting some more out-of-the-box thinking, there is no chance that you might have some dynamic url linking that could be opening the /add/household path?