I set the following Process Fields tag & filename scanner options:

Do not convert
Do not change capitalization
Do not change word separators

As expected, these settings have been written to my configuration file:

$ grep ^pf_ .config/easytag/easytagrc
pf_convert_into_space=0
pf_convert_space=0
pf_convert=0
pf_convert_all_uppercase=0
pf_convert_all_downcase=0
pf_convert_first_letter_uppercase=0
pf_convert_first_letters_uppercase=0
pf_detect_roman_numerals=0
pf_remove_space=0
pf_insert_space=0
pf_only_one_space=0

Now, when I close easytag and open it again, the radio buttons in the "Process fields" settings are set to the following:

Convert _ and %20 to spaces
Capitalize all
Remove spaces

This only happens with the "Do not convert"/"Do not change" settings. The other options are preserved correctly.