Using multiple targets in uV forces full rebuild every time

We are using uV 5.31 and MDK.  We have a project developing the software for an embedded device - this software includes bootloader, application software, FCC test code, UL test code, possibly one or 2 others.  All are based on the same framework packages and such, and share various modules we wrote.  Because of that, it seemed simplest to make each of these a different target in the same project, with each target having its own source group.  The contents of those source groups all draw from the same source directories and there is a good amount of overlap - some files are exclusive to a single group, but some are used by all, and there are various combinations in between.  All the targets use the same compile options, but some use different target memory areas, which shouldn't affect the object files, just the link.  Because of that and the fact that all share a UVBuild directory, I assumed that sources would build as necessary to common (shared) .o files, and the multiple targets would behave the same as a single target with respect to rebuilds - don't rebuild if the source file (and its dependencies) didn't change.  But it doesn't.  Just the presence of a second target (and/or source group) forces each file in the source group of the current target to have its object file renamed  and to rebuild each time.  (The framework packages do not rebuild.)  Searching about this, I find reference (years ago) to this happening if the target option "Use Cross-Module Optimization" set, but this is not set.  Is there some other option causing this behavior or some configuration change we can make to fix it?