Usergroup A and B both have write access to an asset page.
I've created a workflow with two steps.
Step 1. - Approval from usergroup A.
Step 2. - Approval from usergroup B.
Once I'm passed step 1 though, usergroup A doesn't have any access to the asset, can't read, can't do anything.
Is there a reason for that?
Cheers
Yup. Once their portion of the workflow is complete, they are locked out of the asset until workflow is finished. There is a rationale for this, but for the life of me I can't remember what it is right now… 
I'm sure one of the Matrix Developers will remember.
Just by curiosity, can anybody explain the rationale behind it?
It doesn't seem to happen when, once an asset is live, it gets put back into the workflow for review purposes.
Nobody can remember, which probably means that there was none - but was instead a spur of the moment decision that was not reviewed.
This should be changed to block write access but still allow read access to the asset.
Should I lodge a bug report?
Yeah, go for it.