When Marshmallow came out for the S5 I upgraded. When I did every time I restarted my phone things like permissions for apps in the accessibility settings and apps I've set as defaults in the settings would be forgotten. A few days ago I did a factory reset on the phone and upgraded to Marshmallow again and the problem still persists. I think that rules out a particular app doing this. Has anyone else had this issue? Is there a fix?
Samsung broke the defaults about a year ago (fixed it for the S7),it's a Sammy issue,not a Marshmallow or Google issue.
Try using the app Better Open With,or a similar app,from the Google Play Store:
https://play.google.com/store/apps/details?id=com.aboutmycode.betteropenwith&hl=en
As for permissions,that's one of the nice things about Marshmallow,you can fine-tune permissions on a per-app basis:
Go to Settings > Applications > Application Manager.
If need be,tap on the menu icon in the upper L/H Corner to show all apps,user & system.
Then one-by-one,take a look at the permissions & fine tune to your preference,or,until you break functionality.
I believe you can also access permissions (somewhere within the settings) & grant/deny on a permission basis & you'll be presented with which apps have that particular permission.
Either way,it accomplishes the exact same objective.
Also,if you do a total factory reset without restoring any sort of data backup whatsoever,you'll be prompted to grant/deny permissions for each app as you use them for the 1st time after the reset.Afterward, the permissions remain stable until you decide to change 'em.