There appears to have been a change in how Jira treats accented characters in custom fields and user names in between versions 7.0 and 8.6, as the customer is reporting a regression issue.

If the user starts with the accented character, like "Ípek", the user picker does not detect it at all.
However, the same was working fine in 7.0.

Workaround:
Jira 7.0 was using old Java version java 1.8_11.
When using the same old Java version 1.8_11 with Jira 8.6 instead of Java version 1.8_202, it is possible to select the user.