After having several issues with adaptive features on 12.1 databases, in some cases, the quickest way to solve problems was to simply set ‘optimizer_adaptive_features’ parameter to FALSE.
In 12.2, this parameter became obsolete and was replaced with two different parameters : ‘optimizer_adaptive_plans’ and ‘optimizer_adaptive_statistics’. Therefore, it is recommended to apply Patch 22652097 (PROVIDE SEPARATE CONTROLS FOR ADAPTIVE PLANS AND ADAPTIVE STATISTICS FEATURES) in order to have the same behaviour in 12.1.
I read thoroughly the README file of Patch 22652097 and applied it on my 12.1 homes. But when I tried to start the databases afterwards, some of them returned the following errors :

ORA-01078: failure in processing system parameters
ORA-32003: error occured processing parameter 'optimizer_adaptive_features'
LRM-00101: unknown parameter name 'optimizer_adaptive_features'

Of course, I forgot to remove the newly obsolete ‘optimizer_adaptive_features’ parameter in their spfiles. Thus, I quickly fixed the incriminated spfiles and setted, if needed, the two new parameters ‘optimizer_adaptive_plans’ and ‘optimizer_adaptive_statistics’. Finally my databases started correctly.

I now have to apply this patch on other servers, and I surely won’t forget to check if some 12.1 databases have this parameter specified in spfile :

select VALUE from V$SPPARAMETER where NAME='optimizer_adaptive_features' and ISSPECIFIED='TRUE' ;