Please note that there are known issues, some of them beyond our control and due to limitations on Atlassian's part.
Cannot edit Custom Fields on transition screens and create/edit subtask
Custom fields Cannot be used on Jira service management customer portal
Options configuration Size Limitation
Options configuration Size Limitation
The storage limit for all custom field configurations is 128KB or 131,072 characters. Therefore, if you have a large number of entries in MLCS, you may reach a point where you cannot add more.
If you encounter this limit, we suggest reducing the number of options or abbreviating their labels if possible.