I had to netinstall 3 2.9.18 to 2.9.25 upgrades this week on RB112's. I did not change the file or redownload it, it worked on 3 of them, and 3 of them it failed on. Some I used FTP, others I used Winbox drag and drop, and it didn't seem to matter (3 good ones were combinations of both also), and 1 was lost with a direct ethernet connection.
All the same downloaded file. Does MT have a checksum on the NPK file, or does it blindly install it?