Media Upload: -1 appended to brand new filename
There appears to be an edge case where when a new filename ends with a pattern like -NUMBERxNUMBER (e.g. -9x9), a -1 will get appended to the filename as it was a duplicate.
To reproduce:
- name a file something like
unique-filename-3x3
- upload it to the Library
Tested on a fresh install:

Hi there, welcome to WordPress Trac! Thanks for the report.
This is done intentionally in [46822] / #42437, to prevent name collisions with existing or future image sub-size file names.