So. Good news. If you disable the plugin "SceneFormation", your problem should be solved for Jumble Strikers. Long tap the game icon -> Edit plugins -> tap on SceneFormation -> tap DISABLE on the popup -> tap SAVE on the bottom right. If done correctly, you should get a "plugins.js is saved" popup.
This plugin adds a fancy screen for the formation menu, allowing you to add and remove membres of the party, hence making you able to play with only 1 party member if you wish.
The problem is apparently that it also have a Formation battle command, which doesn't seems to be used (at least at the game beginning) which, I guess from the functions in the plugin, allows you to change your party during a battle. I Guess the bugs in battles comes from those functions. So I recommend you to disable the plugin, and enable it only when you need it outsider of battles. But tbh, I'm not sure this plugin is really useful.
Bad news is... It's probably only for this specific game. For another one, it might be a completely different plugin at cause.