Celestia was fully aware of the risks that came with accepting this undertaking. She was fully aware that allowing Chrysalis back into not only Equestria, but into Canterlot, could easily backfire if she wasn't careful. She was aware of the panic that would ensue if word of Chrysalis' presence were to get out into the city, and she was aware that she needed to remain absolutely vigilant if things were to run smoothly.
What she wasn't aware of was that two females spending extended periods of time with each other, even if it was for official business, could be very easily seen as something else.
It likely doesn't help that their actions to follow won't exactly dissuade others from this idea.
It definitely doesn't help that Chrysalis herself is just a bit too happy to 'discuss things' with the princess.
"Diplomatic relations", indeed.