Existing connections with Priority Group Activation - minimizing client impact, timeouts, etc.

Did you check this article.

Beginning in BIG-IP 11.0.0, newly-available pool members in a priority group are not counted toward the minimum-up member requirement for a pool until after the Slow Ramp Time for the pool member has expired.

This means, your pool member A which had the highest priority would not be considered here. Slow ramp is enabled by default with 10 sec. Try disabling the slow ramp setting.

Slow ramp is a feature where the newly available member should not be bombarded with traffic immediately but instead till the slow ramp time is reached.

Also persistence do come into picture - Refer Article.

When a virtual server references a priority group pool, persistence may cause traffic to persist to a node in a lower priority group even after a higher priority group is reactivated.