Is the injury happening because the wrist is bending when you strike? Or is it just from repeated beatings on the bag?
If it is because the wrist is bending on some strikes, slow it down and focus on your technique a little more. Poor form or a punch thrown wrong can seriously tweak a wrist.
Also, you may want to try taping it before you wrap it for a little extra support.
Not sure what kind of gloves you mean by MMA training gloves (the extra padded MMA style gloves for sparring?) but a lot of the guys I train with use either 14oz or 16oz gloves for the heavy bag. Personally, I'm retarded and I just use my wraps.
If this is a "wrist is taking a beating too many times" type thing. Scale it down. Focus some heavy bag days on foot work and kicks, foot work and speed strikes. Take a little of the strain off the wrist and give it some time to recover from the hard sessions on the bag.
Well, I'm not sure if any of that will help, but it might!
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