Check if you have watermark variable on datacard.
Do you have an automatic workflow transition resetting the variable?
Yes i have the watermark variable on the data card.
PDM standard has no automatic transitions.
I think my issue was one of the transition actions didn't have the full list of file types.
Missing an action on one of my transitions was confusing me.
I went round the transition loops over and over and behavior didn't seem to be consistent...
But, I'm also of the opinion that rebooting my machine made this work more predictably.
Anyway it seems to be working as intended now.