One can never test enough - literally.
The STM8 eForth test automation in Travis-CI tests the background task, but it doesn't test its cadence. Maybe that can be done, and maybe it would have been worth doing that. I could have done a manual test - just flashing a W1209 would have shown a flickering LED display - but I didn't. You live and learn...
@Andrew Clapp first discovered that something was wrong. I then identified the cause: a classic "don't use LDW for timers"-error. I first thought that it only hits STM8L devices but also STM8S, this means the whole release was affected. A quick bug-fix release therefore is the best option.
@Eelco the praise came too soon - please check if the 2.2.25 release still solves your problem!
On the plus side: I added INTRX for buffered console RX to the library. Compilation to RAM at 115200 baud really feels much faster than at 9600 baud!