TCP and Reselect Action On Service Down

There are a number of caveats to using 'reselect'. As you have indicated, pool members will not have any knowledge of a connection so sending a packet mid-flow will likely result in a rejection by a server. For this reason reselect can only be used for virtual servers that have address translation disabled or for pool members that are transparent - connection wise

Have a look at the following Article: https://support.f5.com/csp/article/K15095

Note: When choosing the Reselect option for Action on Service Down, the BIG-IP system, when reselecting a pool member, does not re-establish existing TCP connections, but continues the existing connection midstream. If the back-end pool members are not transparent devices, and the virtual server has address translation enabled, all existing TCP connections sent to a pool member will likely reset due to the pool member having no record of these ongoing connections in its connection table. This is analogous to choosing the Reset action, except the pool members will be resetting the connections instead of the BIG-IP system. F5 highly recommends that you choose a different Action on Service Down option, if you do not meet the previously listed criteria for the Reselect option.