Both seats were vacant on my last MIA-LHR in F. I am currently working up to my EXP on AA, as I already requalified for BA Gold this year, so I couldn’t book 1K on my AA Platinum status. My other half was in 2K and I was in 2A. The centre pairs were both taken. There were obvious workarounds (changing status at checkin - yes I know. I had a blonde moment)

Still didn’t open at T-72. Still nothing at t-48 so I called You First. BA said the flight went under airport control already, both seats were now blocked off, and this could only be looked at, at checkin. Nothing could be done now.
At check in they said the seats were still blocked off, for social distancing apparently, in First as it was pretty full. 1K was still empty on boarding and the crew were happy for me to switch across to 1K, so we could sit together on the same side of the plane.

I don’t know if this is a new thing, or just an anomaly of my flight last week, but bear in mind, in case it doesn’t free up at T-72.