From what you are describing I'm guessing the small tacks used to fit backing bars have broken clean through as a result of distortion caused by the groove weld. That isn't unusual to find and normally the broken weld can't propagate into the base metal. So I see 2 views here.
From a practical point of view, the cracks likely won't cause any difficulties other than they don't look good, and not many people will see them. If you have them removed, the contractor will probably cut them with a cold chisel (if they are small) in which case you have the same as before except with a wider gap. Or someone will attack the tacks with a grinder and you could end up with notches and cuts that are worse than the original problem. Often the owner will side with the contractor in leaving the tacks as is.
From a code point of view, D1.1 has provisions (5.18.2.3) that tack welds not incorporated into the final welds shall be removed, except in statically loaded structures they can remain unless removal is required by the engineer. Also, 5.18.2 subjects tack welds to the same quality requirements as the final welds (ie: no cracks).
If you are the engineer, you can require tack removal, cracked or not.
If you are the inspector, the quality isn't there. But I would make sure I consulted with the Owner/Engineer for their view before I told the Contractor to do address them. Hopefully, you can get it in writing.
Have a happy
CHGuilford
Were the tacks holding the backing in place from the weld side or the back of the joint? When did you note the cracks, after the final weld or before? Did the tacks crack down the center or tear out of the base metal?
If these were after the completed weld and have created defects in the base metal then you'll have to have them repaired. If this is a continuous problem, specify where you want the tacks so they HAVE to be incorporated into the weld.
If you need to quote the code book look under D1.1 - Fabrication sect. 5.10 Backing which refers to lengths required, thicknesses and lengths of welds required for cyclically and statically loaded structures. Otherwise depend on the acceptance criteria for discontinuities and defects.
TAB