I posted this on the ST forums and the reply doesn't work, and that's the end of it - the place gets so many posts, mostly never solved...
Clearly there are multiple configs for this around the place. The file config.txt, which in my case is found at
C:\ST\STM32CubeIDE_1.4.0\STM32CubeIDE\plugins\com.st.stm32cube.ide.mcu.externaltools.stlink-gdb-server.win32_184.108.40.206101291314\tools\bin has a value in it but it is commented-out, and anyway has no effect.
The default value is 31 but it somehow got changed to 1 when I was trying different debugger settings (speeds, jtag or sw, etc).
There is only the one config.txt file in the whole \ST directory.
Many thanks for any ideas :) Not a huge deal, but a slight irritation to lose the debug output during loading.
Currently I am using STLINK V3, which runs OK at 24MHz and fairly solidly at 8MHz. The ISOL version (an extra board) runs ok at 8MHz. It was changing from STLINK V2 to STLINK V3 (which *is* a lot faster) which produced this mysterious change.