Add an auto incremented value to an existing ID

Assuming maintenance_ID field is already given, I would use Add autoincremental field tool (in Processing Toolbox > Vector table) to prepare a temporary field, then put them together.

(1) Start Add autoincremental field and select maintenance_ID as the Group value.

(2) The tool will return a new layer Incremented with a new AUTO field.

(3) Use the Field Calculator to update maintenance_ID field. The expression is concat("maintenance_ID", "AUTO").

(4) Examine the updated attribute table, and delete "AUTO" temporary field.