This likely isn’t a [technical] bug but in case: check in Settings (cog icon in left-hand panel), go to “Files & Links”, and see what you have configured for “Default location for new notes”. The default is “Vault folder” rather than “Same folder as current file”.
However… I think devs should think about the confusion mobile is introducing as opposed to desktop version: As a user, I would expect that if I select a folder (and it is visibly shown as selected, which doesn’t happen on desktop) then tapping the new file icon would create the note within the selected folder.
Hmmm I just tried and doesn’t create note in a selected folder for me. If you have that setting “Same folder as current file” and a note open, I’d assume it creates where open note is; maybe that is what you see?