thank you for joing the forum! I am sorry to hear that you are experiencing issues with your Buy Box rate plumetting.
As you have noticed, there has been a sudden change to Amazon’s algorythim in determinging the Featured Offer criteria as a consequence of anti-trust settlement between Amazon Europe and the authorities in the UK and EU:
This means, that while previously a specific seller was able to maintain the Buy Box on a listing quite easily, now they are mostly rotating the Buy Box if there are multiple sellers with similar metrics or price.
The only thing which now leads to the loss of Buy Box is mostly poor metrics.
My recommendation is that you lower the price - this is something that will be the most likely to give you the Buy Box in current circumstances.
I hope this helps. Please feel free to post here whenever you need help.