UI Actions configured as list-level buttons or list choices do not render on M2M (many-to-many) related lists. This applies to any table that acts as a join/relationship table displayed as an M2M related list on a parent form — including m2m_kb_task. ServiceNow's M2M related list renderer simply doesn't invoke the standard UI Action framework the way a normal list view or regular related list does. This is by design, not a bug.
You can confirm this yourself: if you navigate directly to the m2m_kb_task list view (e.g., via /m2m_kb_task_list.do), the UI Action button will appear there. It only disappears when the same table is rendered as an M2M related list embedded on the Problem form.
Recommended approaches
1. UI Action on the parent form instead. If the action operates on the related records collectively, place the UI Action on the problem table as a form button or form context menu item. From there, your server-side script can query the M2M table using GlideRecord against m2m_kb_task filtered by current.sys_id and perform the needed logic. This is the simplest and most upgrade-safe approach.
2. Convert to a regular related list with a reference field approach. Instead of using the M2M relationship, create a custom child table with explicit reference fields to both parent tables. A regular related list fully supports UI Actions — list buttons, list choices, row-level actions, and context menus all work as expected. This is the cleanest long-term solution if the M2M pattern isn't strictly required by your data model.
