I just tried this out, but I don't see an error.
What I did:
Start mission at 08:30Z, WP1 is about 50NM away.
Set the required TOT for WP in the CDU to 08:39 -> RIAS is showing a speed I might be able to handle
Got to the DTOT adjust and set it to -00:01:00, effectively making my desired TOT a minute shorter (08:38). -> RIAS showing higher speeds
Back to DTOT adjust, and set it to +00:01:00 . Now my DTOT for WP is 08:40 -> RIAS showing lower speeds.
By the sounds of it, you interpret the DTOT Adjust as "The new DTOT". The value you enter there, is a correction to the existing DTOT in hours, minutes and seconds.
Please post a track with you step in case I'm misunderstanding you.