broken display of letters containing 0x81 in their UTF-8 representation

Letters containing the 0x81 byte in their UTF-8 representation, such as U+0141 LATIN CAPITAL LETTER L WITH STROKE (0xC5 0x81: ?) or U+2077 SUPERSCRIPT SEVEN (0xE2 0x81 0xB7: ?), are sometimes displayed brokenly. The breaking is always the same - the 0x81 byte gets replaced with a question mark (0x3F).

This can be observed by creating a contact with such letter (in the default dataset - see contact of cid=4), visiting it (so it gets added to the Recently Viewed list) and then clicking around CiviCRM. In certain screens the name is displayed properly (Contact Summary view), in other screens broken (search selector), and in the Recently Viewed list sometimes properly and sometimes broken.

I'm currently at a total loss of what can change 0x81 to a question mark 'just because' and only sometimes.