Combine text on condition

And another approach. Also interpreting your "line break" criteria to mean you want the items in the same cell separated by a line break, which is different than what you show in your example:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bY5BDsIgFESvMmFdE/QGtR0KCfARPpqm6f2v0brQuHA97+XNthlr7dUMZpKsY8isCLkVThokm3347lKYL4IoL7jOiIeuUDb9ZVJbUDnOkK5/3NSbwo9PQj1DBVOJspLIPd3P8Ee5nYrj2ZiDcyiVrSGGxSvel/YD", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Aircraft number" = _t, #"Maintainance Task" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Aircraft number", type text}, {"Maintainance Task", type text}}),
    
    #"Add isOPEN-O" = Table.AddColumn(#"Changed Type","is Open-O", each Text.StartsWith([Maintainance Task],"OPEN-O"), type logical),
    #"Sorted Rows" = Table.Sort(#"Add isOPEN-O",{{"Aircraft number", Order.Ascending}, {"Maintainance Task", Order.Ascending}}),
    
//Note GroupKind.Local
    #"Grouped Rows" = Table.Group(#"Sorted Rows", {"Aircraft number", "is Open-O"}, {
        {"Maintainance Task", each if [#"is Open-O"]{0}
                        then Text.Combine([Maintainance Task],"#(lf)") 
                        else Text.Combine([Maintainance Task],"|"),type text}
        },GroupKind.Local),
        
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"is Open-O"})
in
    #"Removed Columns"

Result

If you really want the OPEN-O's in separate cells/rows, then you can use this code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bY5BDsIgFESvMmFdE/QGtR0KCfARPpqm6f2v0brQuHA97+XNthlr7dUMZpKsY8isCLkVThokm3347lKYL4IoL7jOiIeuUDb9ZVJbUDnOkK5/3NSbwo9PQj1DBVOJspLIPd3P8Ee5nYrj2ZiDcyiVrSGGxSvel/YD", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Aircraft number" = _t, #"Maintainance Task" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Aircraft number", type text}, {"Maintainance Task", type text}}),
    
    #"Add isOPEN-O" = Table.AddColumn(#"Changed Type","is Open-O", each Text.StartsWith([Maintainance Task],"OPEN-O"), type logical),
    #"Sorted Rows" = Table.Sort(#"Add isOPEN-O",{{"Aircraft number", Order.Ascending}, {"Maintainance Task", Order.Ascending}}),
    
//Note GroupKind.Local
    #"Grouped Rows" = Table.Group(#"Sorted Rows", {"Aircraft number", "is Open-O"}, {
        {"Maintainance Task", each if [#"is Open-O"]{0}
                        then [Maintainance Task] 
                        else {Text.Combine([Maintainance Task],"|")}, type {text}}
        },GroupKind.Local),
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"is Open-O"}),
    #"Expanded Maintainance Task" = Table.ExpandListColumn(#"Removed Columns", "Maintainance Task")
in
    #"Expanded Maintainance Task"

Results2