A classic case of just too many coats. Like tree rings there could 10 different paint films (coats) on there. No wonder its so soft, filler and paint aren't exactly structural components.
First base coat in red is reminiscent of Bondo or Penetrox Primers from yesteryear, air drying solvent based, very popular with amateur and professionals back in the day. These days the modern equivalent is used by guys fabricating structural steel.
My guess is that its been done a good few times in Cellulose, so relatively soft and fragile. Thick filler is a recipe for disaster, cracking if under pressure, around bolt holes etc. Thinner paint films are relatively flexible when first applied and the guards mounted, so avoiding the problem.
How you proceed is your choice....spot repair and hope for the best, a new/replacement guard finished with modern two pack paint /lacquer system, or strip this guard to bare metal, fearing the worst and being prepared for structural repairs, and a finish to suit.
Heating the paint gently with a hot air gun will soften any filler underneath enough for the whole lot to be easily peeled off with an old chisel, far less hazardous and less chance of damage than attacking it with flap wheels and grinders as you explore what's under the paint.
Swarfy.