I fully agree. Thread lockers no joy. Larger screw might work.
There are several products designed to work in a Hollow wall that might help.
Worst case repair takes careful measurement.. But here it is
Get a couple of metal plates roughly the same size as the "Footprint" of the mounts for the grab bar
DRILL ALL THE WAY THROUGH (Careful if you have a lighted handle you don't drill the wires)
also drill the plates (Same pattern as the mounts.. Three options here
1 Drill and tap I like this, (Thread locker will work on the threads here) and screw a screw into the plate from outside.. then cut off the excess bolt.
2: Reverse that. Drill to pass the bolt. then put a nut on the outside (Threadlocker works or use Nylock nuts) and cut off excess (Dremel)
3: Reverse 2: Drill to pass and use a nut on the inside.
NO way is that going to pull out. though it may still be loose it wont' pull out without leaving one heck of a hole in ye old wall.
On one more very labor intensive and expensive (usually) method I just used elsewhere.
Cut an opening. Slip some nice 2" plank (only a few inches) in there and slide it back and forth so it's well behind solid wall. Glass over it (Fiberglass body filler) paint, re-drill and sink those bolts into SOLID wood instead of the cheap thing stuff the wall is made of.
But that is a whole lot of work (normally.. In my case it was not the grab handle but something else and it was the only way to do it. also due to the size and shape the device bolted on covers the repair so no painting was needed. PAINT 60 dollars last time I bought some. That is JUST the paint.