Looks like the bug is reproduced only in the situations when the «Magento_Customer» module is processed by system before the «Magento_Theme» module.
These modules do not set a processing ordering relative each other, so the bug appears in random cases, when you installed some third-party module which uses some <sequence> rules.
Another effect of the same Magento 2 core bug is: Bug: the order's title is shown at the bottom of a customer account's order page (and my fix)