You are not the only one on that score. The same crack caught my eye on mine, and I couldn’t believe how weak that part is when out of the fender. The back there is made of thinner plastic than it is supposed to be, and so ordinary vibration and bumps only serve to wear at it over a period.
Your JB Weld repair is just the thing many of us have been experimenting with, anything that prevents the flow assists. I could recall how the entire light unit was trembling on the side stand and how I assumed that it was one of the minor quirks of Harley, but it was actually a weak design.
Other guys just swap the trim, and some have had it covered by warranty when the bike was new. It appears to prolong the life of the replacements to pack out the bottom bracket and tighten things down.
I concur with all here that the place ought to have been made twice as thick in the factory. Until Harley alters it, stiffening it is as well as we can do.
Thanks for posting the photo and the fix, practical hints such as that of the real world are what make this forum so valuable.