Strange "Invalid UTF-8" output when an input file is attached

[midn1]

I'm trying to feed in binary data through the terminal and am expecting binary data in return. Apparently setting any input file, whether /dev/stdin or the source file itself, makes something along the way expect UTF-8 text. That something then spams [Invalid UTF-8] for each wrong byte. This cannot be the terminal I'm running, since it occurs when redirecting the output to a file, too. I admit it's a strange use case, but so is Brainfuck in the first place.

Minimal reproducible example under Debian Bullseye:

test.bf:

    -[.-]

In terminal:

$ beef test.bf 
��������������������������������������������������������������������������������������������������������������������������������~}|{zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;:9876543210/.-,+*)('&%$#"! �


       
$ beef -i test.bf test.bf 
[Invalid UTF-8] \xff[Invalid UTF-8] \xfe[Invalid UTF-8] \xfd[Invalid UTF-8] \xfc[Invalid UTF-8] \xfb[Invalid UTF-8] \xfa[Invalid UTF-8] \xf9[Invalid UTF-8] \xf8[Invalid UTF-8] \xf7[Invalid UTF-8] \xf6[Invalid UTF-8] \xf5[Invalid UTF-8] \xf4[Invalid UTF-8] \xf3[Invalid UTF-8] \xf2[Invalid UTF-8] \xf1[Invalid UTF-8] \xf0[Invalid UTF-8] \xef[Invalid UTF-8] \xee[Invalid UTF-8] \xed[Invalid UTF-8] \xec[Invalid UTF-8] \xeb[Invalid UTF-8] \xea[Invalid UTF-8] \xe9[Invalid UTF-8] \xe8[Invalid UTF-8] \xe7[Invalid UTF-8] \xe6[Invalid UTF-8] \xe5[Invalid UTF-8] \xe4[Invalid UTF-8] \xe3[Invalid UTF-8] \xe2[Invalid UTF-8] \xe1[Invalid UTF-8] \xe0[Invalid UTF-8] \xdf[Invalid UTF-8] \xde[Invalid UTF-8] \xdd[Invalid UTF-8] \xdc[Invalid UTF-8] \xdb[Invalid UTF-8] \xda[Invalid UTF-8] \xd9[Invalid UTF-8] \xd8[Invalid UTF-8] \xd7[Invalid UTF-8] \xd6[Invalid UTF-8] \xd5[Invalid UTF-8] \xd4[Invalid UTF-8] \xd3[Invalid UTF-8] \xd2[Invalid UTF-8] \xd1[Invalid UTF-8] \xd0[Invalid UTF-8] \xcf[Invalid UTF-8] \xce[Invalid UTF-8] \xcd[Invalid UTF-8] \xcc[Invalid UTF-8] \xcb[Invalid UTF-8] \xca[Invalid UTF-8] \xc9[Invalid UTF-8] \xc8[Invalid UTF-8] \xc7[Invalid UTF-8] \xc6[Invalid UTF-8] \xc5[Invalid UTF-8] \xc4[Invalid UTF-8] \xc3[Invalid UTF-8] \xc2[Invalid UTF-8] \xc1[Invalid UTF-8] \xc0[Invalid UTF-8] \xbf[Invalid UTF-8] \xbe[Invalid UTF-8] \xbd[Invalid UTF-8] \xbc[Invalid UTF-8] \xbb[Invalid UTF-8] \xba[Invalid UTF-8] \xb9[Invalid UTF-8] \xb8[Invalid UTF-8] \xb7[Invalid UTF-8] \xb6[Invalid UTF-8] \xb5[Invalid UTF-8] \xb4[Invalid UTF-8] \xb3[Invalid UTF-8] \xb2[Invalid UTF-8] \xb1[Invalid UTF-8] \xb0[Invalid UTF-8] \xaf[Invalid UTF-8] \xae[Invalid UTF-8] \xad[Invalid UTF-8] \xac[Invalid UTF-8] \xab[Invalid UTF-8] \xaa[Invalid UTF-8] \xa9[Invalid UTF-8] \xa8[Invalid UTF-8] \xa7[Invalid UTF-8] \xa6[Invalid UTF-8] \xa5[Invalid UTF-8] \xa4[Invalid UTF-8] \xa3[Invalid UTF-8] \xa2[Invalid UTF-8] \xa1[Invalid UTF-8] \xa0[Invalid UTF-8] \x9f[Invalid UTF-8] \x9e[Invalid UTF-8] \x9d[Invalid UTF-8] \x9c[Invalid UTF-8] \x9b[Invalid UTF-8] \x9a[Invalid UTF-8] \x99[Invalid UTF-8] \x98[Invalid UTF-8] \x97[Invalid UTF-8] \x96[Invalid UTF-8] \x95[Invalid UTF-8] \x94[Invalid UTF-8] \x93[Invalid UTF-8] \x92[Invalid UTF-8] \x91[Invalid UTF-8] \x90[Invalid UTF-8] \x8f[Invalid UTF-8] \x8e[Invalid UTF-8] \x8d[Invalid UTF-8] \x8c[Invalid UTF-8] \x8b[Invalid UTF-8] \x8a[Invalid UTF-8] \x89[Invalid UTF-8] \x88[Invalid UTF-8] \x87[Invalid UTF-8] \x86[Invalid UTF-8] \x85[Invalid UTF-8] \x84[Invalid UTF-8] \x83[Invalid UTF-8] \x82[Invalid UTF-8] \x81[Invalid UTF-8] \x80~}|{zyxwvutsrqponmlkjihgfedcba`_^]\[ZYXWVUTSRQPONMLKJIHGFEDCBA@?>=<;:9876543210/.-,+*)('&%$#"! �

This also happens when piping in input like so: beef test.bf < test.bf