REAPER doesn't seem to like it if two VST built-in presets have the same name.
Steps to reproduce:
1. Download and install
Pianoteq demo (tested with v5.7.1).
2. Load "VSTi: Pianoteq 5 (Modartt) (5 out)" in REAPER (tested in v5.25+ x86 and v5.25 x64).
3. Select preset "D4 Classical Recording" from REAPER's VST built-in programs downdown list. Notice that there are two presets with this name, select the first one. This will select the "D4 Classical Recording BA" preset in Pianoteq's GUI.
4. Now select the second "D4 Classical Recording" from the list. This will select the "D4 Classical Recording AB" preset. However, in REAPER's dropdown list the first one is still selected (I guess because REAPER selects by name rather than by index).
This is a very long-standing bug (I seem to remember reporting this during v4.0 development), so I guess it's not really much of a problem, but it would be nice to have this fixed eventually.
Note that Pianoteq could also fix this by using unique VST2 (23-char) preset names. However, I have seen the same with a bunch of other VSTs, so it would be good to fix in REAPER if somewhat possible.