Hello G.,
As mentioned by @albert, could you provide your example?
Because, with the last version (1.2021.2) and this code:
@startgantt
[Task1: Prototype design] as [T1] lasts 13 days
note bottom
note for task1
more notes
end note
[M1] happens on 5 days after [T1]'s end
[M1] displays on same row as [T1]
@endgantt

All seems OK.
Regards,
Th.