Hi Spartan,
I can duplicate the same issue you are seeing. My steps for reproduction are:
New IG army,
Add infantry platoon, (shows as 130 points)
Tick "add heavy weapons team" (listed as 5 points. Actually reduces the total points to 125)
The heavy weapons team should be outfitted by default with a mortar and shouldn't require additional weapon selection, but even so, adding an autocannon only boosts the point total back up to the original 130, as opposed to the 145 that it actually should be (i think thats right. I don't have my Codex in front of me right now).
I have AB version 3.2a, warhammer 40k 5th ed files version 1.3.
I took a look at the entry in ig5UniDatV2.dat, which APPEARS to be the right IG file (there are about 4 in there, most appear to be old codex for some reason), and for the entry ig5E_HWTeA under the infantry squad upgrades, there is a Text costs of 5+, but there is no actual cost added. I believe what is happening here is that since the squad is 10 people @ 50 pts, its technically 5 pts/guy. This is removing one guy and turning another guy into a "heavy weapons team", so you're getting 5 points back and it is costing you nothing.