Unexpected index scan instead of seek on update
Since there is no index on (tasklockedby) or any index that has that column in any column position, the only option for the optimizer to find which rows need to be updated is to do a table scan (so clustered index scan in this case).

I don't know why an index suggestion was not made.

I would suggest a simple index on (tasklockedby), without any INCLUDE columns, if you want to improve the efficiency of the UPDATE statement. If you have other (select) queries that need to get more columns in the result set, then you many be better with a different one.