forcemerge 42801 40853
quit

Hello Michelangelo,

Thanks for your bug report.  This was fixed in:

commit 5c39f6f1165a33f5714eedd3a413f786dffbb5a2
Author: Mauro Aranda <maurooaranda@gmail.com>
Date:   Fri Aug 14 16:40:40 2020 +0200

    Make sure we only act on edited widgets in Custom-save

    * lisp/cus-edit.el (Custom-save): Only act on edited widgets in the
    buffer.  If we attempt to redraw all widgets, we confuse
    custom-variable-modified-p, or we end up drawing State buttons for
    all options, including the hidden ones (bug#42801).

The error you posted was custom-variable-modified-p being confused.