Some have reported that they have lost their seat selection at some time after dropping down; especially if there is an intervening change made to the booking or the flight. Others have reported that they have not lost their seat selections.
See these threads among others:
https://www.flyertalk.com/forum/brit...ly-2019-a.html
https://www.flyertalk.com/forum/brit...op-status.html
https://www.flyertalk.com/forum/brit...18-update.html
https://www.flyertalk.com/forum/brit...questions.html