I’m not a DocManager expert, but it looks like you forgot to fill in the "Ignore These TVs" field (which, IMO, should be a set of checkboxes marked "Change these TVs"). Opt-in is almost always friendlier and safer than opt-out.
If I’m right, DM should probably do a better job of warning you about what can happen if you leave that field blank.
TBH, I don’t know what the intended behavior is, but DM probably shouldn’t overwrite a TV that has been set with a blank value unless the user selects or enters something in that TV in DM. I’ve never used the TV part of DM, so I can’t tell what the "normal" behavior is.
None of which helps you. I hope you have a recent backup or at least can remember what the settings are.
Note that you can not only set a TV’s Default Value field to @INHERIT to have it inherit the value from a parent or ancestor, but that you can still specify a default value with something like:
@INHERIT Some Default Value
IIRC, if you change this in the TV itself, you’ll still have to save all the documents to have it take effect.
Sorry for your loss.