AVR128DA48 and AVR128DB48 Curiosity Nano boards don't work on 1.5.8 or later

[technoblogy]

I have an application (uLisp) that used to work on the AVR128DA48 Curiosity Nano or AVR128DB48 Curiosity Nano with DxCore versions up to and including 1.5.6. However, it doesn't work on 1.5.8, or the latest, 1.5.11.

It just hangs up when I open the Serial Monitor, rather than displaying the usual text prompt.

It uses the default settings on the Tools menu, apart from:

Write to flash from app: "Above 96K (AVR128 only)"

I use PROGMEM, but not PROGMEM_MAPPED.

I use Serial, and cater for the differences on these boards with, on the AVR128DA48 Curiosity Nano:

#define Serial Serial1

or, on the AVR128DB48 Curiosity Nano:

#define Serial Serial3

I noticed that the DxCore versions it stopped working on are the ones with the new How to set FLMAP option, but I've tried every option on that menu and none of them solve the issue.

I'm using Arduino IDE 1.8.19 on MacOS 10.13.

Any suggestions?

Thanks, David