There is currently a known issue which has caused some players to be placed in Spectator Mode for the current war after leaving and rejoining their alliance. We are investigating the issue here and hope to get it resolved as soon as possible. Thank you for your patience and our apologies for the inconvenience caused by this!
[EDIT 02 Dec 2020 at 14:20 (UTC+0)] We have now investigated this issue here. We are currently working on implementing a fix before the next War. As this issue affected a very small amount of players, we are unable to restore affected players to being active war participants for the current war, as doing so has the potential for unseen consequences. We do apologize for any inconvenience caused by this. If you were one of the players who experienced this issue, please contact Support through the in-game link for further assistance.
[EDIT 04 Dec 2020 at 14.00 (UTC+0)] This issue has now been resolved. Thank you again for your patience while this was investigated and resolved.