regex as $filter in projection

As per @zangw's answer,

this ISSUE-SERVER-8892, According this issue Use $regex as the expression in a $cond, the $regex could NOT be used with cond for current mongo version.

MongoDB v4.1.11 has launched new features in ISSUE-SERVER-11947, This features adds three new expressions $regexFind, $regexFindAll and $regexMatch to the aggregation language.

In your example, You can use $regexMatch expression,

Model.aggregate([
  {
    $project: {
      tags: {
        $filter: {
          input: "$tags",
          cond: {
            $regexMatch: {
              input: "$$this",
              regex: query
            }
          }
        }
      }
    }
  }
])

Playground

According this issue Use $regex as the expression in a $cond, the $regex could NOT be used with cond for current mongo version.

Maybe you can try this one, filter the area3 through $match, then get all matched tags through $group, then remove the _id through $project.

caseNote.aggregate([{$unwind: '$tags'},
               {$match: {tags: /area3/}},
               {$group: {_id: null, tags: {$push: '$tags'}}},
               {$project: {tags: 1, _id: 0}}])
    .exec(function(err, tags) {
        if (err)
            console.log(err);
        else
            console.log(tags);
    });

Results:

{ "tags" : [ "C area3", "b_area3" ] }