By removing drv:, you are causing the qualified path parser to discard ignition-ussfa1igntpap01:default. (Really, causing that string to be considered a qualifier.)
Since the :/tag: fragment supplies a tag path, and a tag path is usually sufficient to obtain the parts supplied by :/drv, it is working by chance.
If you then edit the pen in the PowerChart, it likely is "fixing" the drv to be syntactically correct. You should inspect that, and compare to the part you are forcing it to ignore. I suspect a case mismatch. Quite likely the leading I in Ignition.