When “trashing” a message, it is moved to a trash-folder (i.e.,
mu4e-trash-folder) and is flagged as “trashed” (a “T” is added to its
filename).
Following the Maildir specification, this flag is to be interpreted as “marked for manual removal later”. However, some mail retrieval and IMAP synchronization programs interpret this instead as a trigger for immediate removal.
For such cases, you can influence mu4e’s behavior; see
mu4e-trash-without-flag.