I rigorously observed these procedures when I last caulked, which was after
a completely new installation of the tub wall. So, the only variable
left to me is the brand/type of caulk.

Regards,
-----------------
You're not the only one in this boat. The claims of mildew resistance,
in my experience, is wishful thinking on the Mfg.'s part.

What I know do is I keep a spray bottle of 10:1 bleach (10 oz water to
1 oz bleach) by the tub. After I get out, a give a couple of quick
mists around the caulk lines. Been good for 2 years now.