Steps to reproduce:

Start fish shell;
Type тест, press up arrow;
Crash.

Error displayed after crash:

error: /builddir/build/BUILD/fish-3.6.0/src/reader.cpp:431: failed assertion: offset != wcstring::npos && "Should have found a match in the search result"

Backtrace from gdb after installing debug symbols:

#0  0x00007ffff79cfe5c in __pthread_kill_implementation () from /lib64/libc.so.6
#1  0x00007ffff797fa76 in raise () from /lib64/libc.so.6
#2  0x00007ffff79697fc in abort () from /lib64/libc.so.6
#3  0x000055555557907b in __fish_assert (msg=0x5555556ac790 "offset != wcstring::npos && \"Should have found a match in the search result\"", file=0x5555556a9e18 "/builddir/build/BUILD/fish-3.6.0/src/reader.cpp", line=431, error=<optimized out>)
    at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/common.cpp:1860
#4  0x00005555555dc4e9 in (anonymous namespace)::reader_history_search_t::append_matches_from_search (this=0x5555557b2750) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:431
#5  (anonymous namespace)::reader_history_search_t::move_backwards (this=0x5555557b2750) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:472
#6  (anonymous namespace)::reader_history_search_t::move_in_direction (dir=history_search_direction_t::backward, this=0x5555557b2750) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:497
#7  reader_data_t::handle_readline_command (this=0x5555557b2070, c=<optimized out>, rls=...) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:3715
#8  0x00005555555deb6c in reader_data_t::readline[abi:cxx11](int) (this=0x5555557b2070, nchars_or_0=<optimized out>) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:4484
#9  0x00005555555e4788 in read_i (parser=...) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:3201
#10 reader_read (parser=..., fd=<optimized out>, io=...) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/reader.cpp:4749
#11 0x000055555556924c in main (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/fish-3.6.0-1.fc37.x86_64/src/fish.cpp:576

Reproducable: always.

Fish version: 3.6.0, from Fedora package fish-3.6.0-1.fc37.x86_64.

When testing in clean environment using command sh -c 'env HOME=$(mktemp -d) fish' it could be reproduced as follows:

Run sh -c 'env HOME=$(mktemp -d) fish';
Enter command echo Тест and hit Enter (notice capital Т);
Type тест and press up arrow key;
Crash.

Looks like it's something wrong when searching history case insensitive.